Functionality of surfactants in waste-activated sludge

Science of the Total Environment 609 (2017) 1433–1442

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Science of the Total Environment journal homepage: www.elsevier.com/locate/scitotenv

Review

Functionality of surfactants in waste-activated sludge treatment: A review Renpeng Guan a,b, Xingzhong Yuan a,b,⁎, Zhibin Wu a,b, Hou Wang a,b,c, Longbo Jiang a,b, Yifu Li a,b, Guangming Zeng a,b a b c

College of Environmental Science and Engineering, Hunan University, Changsha 410082, PR China Key Laboratory of Environment Biology and Pollution Control, Hunan University, Ministry of Education, Changsha 410082, PR China School of Chemical & Biomedical Engineering, Nanyang Technological University, Singapore 639798, Singapore

H I G H L I G H T S

G R A P H I C A L

A B S T R A C T

• Sludge EPSs are affected by micelles formed by surfactant molecules. • Water is released from sludge by surfactants that promote the dissolution of EPSs. • VFA and methane yields improved via surfactant enhancement of hydrolase activity. • Pollutants contained in sludge can be removed effectively by surfactants.

a r t i c l e

i n f o

Article history: Received 14 May 2017 Received in revised form 21 July 2017 Accepted 21 July 2017 Available online xxxx Editor: Jay Gan Keywords: Waste-activated sludge Surfactants Dewatering Anaerobic digestion Organic pollutants Heavy metals

a b s t r a c t Proper treatment of waste-activated sludge (WAS) involves three pivotal processes, dewatering, anaerobic digestion, and pollutants removal, which need to be re-assessed urgently. Although many traditional sludge treatments have been developed, it is prudent to enhance the efficiency of sludge treatment using multifunctional, flexible, and environmentally friendly surfactants. With regard to sludge dewatering, surfactants can weaken the binding interaction between sludge flocs and promote the dissolution of extracellular polymeric substances (EPSs), resulting in the release of bound water. Using surfactants in anaerobic digestion promotes the release of enzymes trapped in sludge and improves the activity of enzymes during hydrolysis. Owing to their characteristic encapsulation of hydrophobes into self-assembled aggregates (micelles), surfactants can form host-guest complexes with polycyclic aromatic hydrocarbons (PAHs). Additionally, surfactants can enhance the desorption of heavy metals and prevent the emergence of heavy metal residue. This review summarizes the current surfactant-based sludge treatment technologies according to their roles in sludge disposal solutions. Then, possible mechanisms of surfactants in sludge dewatering, anaerobic digestion, and the removal of organic pollutants and heavy metals are analysed systemically. Finally, changes to sludge treatment via the aid of surfactants are highlighted. This review presents the comprehensive advances in the use of surfactants in WAS reduction, recycling, and risk relief, underscoring their roles in increasing economic efficiency and ensuring environmental quality. © 2017 Elsevier B.V. All rights reserved.

⁎ Corresponding author at: College of Environmental Science and Engineering, Hunan University, Changsha 410082, PR China. E-mail address: [email protected] (X. Yuan).

http://dx.doi.org/10.1016/j.scitotenv.2017.07.189 0048-9697/© 2017 Elsevier B.V. All rights reserved.

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R. Guan et al. / Science of the Total Environment 609 (2017) 1433–1442

Contents 1. 2.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . Surfactants in waste-activated sludge dewatering . . . . . . . . 2.1. Surfactants in traditional waste-activated sludge dewatering 2.2. Combined treatment for dewatering waste-activated sludge 3. Surfactants in anaerobic digestion. . . . . . . . . . . . . . . . 4. Surfactants in organic contaminant decontamination . . . . . . . 5. Surfactants in heavy metal removal . . . . . . . . . . . . . . . 6. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . Appendix A. Supplementary data . . . . . . . . . . . . . . . . . References. . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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1. Introduction During the process of activated sludge wastewater treatment, a large amount of excess waste-activated sludge (WAS) is generated, which can cause serious environmental pollution. Sludge is an extremely complex, heterogeneous, colloidal body composed of organic debris, bacteria, inorganic particles, etc. Generally, the water content (Wc) of sludge is between 99% (before thickening) and 95% (after thickening). This can lead to difficulties in further treatments and utilization of sludge (Appels et al., 2013). Furthermore, the large amount of organic matter (OM) in sludge has not been utilized effectively. In addition, many hazardous substances, such as polycyclic aromatic hydrocarbons (PAHs), dyes, heavy metals, antibiotic, pesticide, and oil, are contained in sludge (Mangas et al., 1998; Wilson and Jones, 1993). Among these, PAHs, dyes, and heavy metals are the most toxic and dangerous pollutants based on their potential for environmental and human health risks (Wang et al., 2015; Wang et al., 2016; Wu et al., 2017b, 2016; Yuan et al., 2016). Hence, the treatment and disposal of sludge is an urgent and arduous task. To date, several methods have been explored to treat sludge and utilize it as a resource, including sludge dewatering, anaerobic digestion, removal of organic pollutants and heavy metals, pelletization for fuel preparation (Jiang et al., 2014, 2015), and liquefaction for biooil refining (Leng et al., 2015a; Yuan et al., 2015a). Among these, sludge dewatering, anaerobic digestion, and removal of organic pollutants and heavy metals are the most important treatments for reducing, recycling, and harmless treating WAS. However, traditional sludge treatment cannot achieve these goals. Hence, improvement of traditional methods is crucial. Surfactants originating from the contraction of a surface active agent represent a number of chemical species that are capable of modifying the interface characteristics between aqueous and non-aqueous liquids (Beneito-Cambra et al., 2013). Ordinarily, surfactants contain a hydrophobic part and a hydrophilic part. The characteristic properties of surfactant molecules are determined by the hydrophilic parts in the molecular structure and the presence of separated hydrophobic parts. The hydrophilic part causes surfactants to become soluble in water; conversely, the hydrophobic part tends to concentrate surfactants on the interface between water and air (Volkering et al., 1997). When surfactants concentrate on the free liquid surface and the interfaces between immiscible liquids, liquid surface tension can be reduced. The properties of wetting, dispersing power, detergency, and solubilization are influenced by changes in surface tension (Olkowska et al., 2011). Surfactants can enhance solubilization and reduce surface tension after absorbing onto the solid-liquid interface. Because of these properties, surfactants are widely used in almost all industries, such as oil, cosmetics, painting, dye, and textiles (Castro and Cirelli, 2005), for better sludge treatment effects. In sludge dewatering, surfactants can improve dewaterability by changing the release of EPSs (Chen et al., 2004). In the presence of

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surfactants, tightly bound EPSs (TB-EPSs) and loosely bound EPSs (LBEPSs) can dissolve into the aqueous phase, which enhances the release of bound water from sludge (Chen et al., 2001). When surfactants are present in anaerobic digestion, the rate of sludge hydrolysis is improved because surfactants accelerate the degradation of OM. Additionally, enzymes trapped in sludge flocs can be released (Zhou et al., 2015). The increase in enzyme activity and the existence of abundant OM can simultaneously improve the efficiency of acidification and methanogenesis. Thereby, yields of volatile fatty acids (VFAs) and methane could be enhanced (Luo et al., 2013). With regard to the removal of PAHs, surfactants can work in cooperation with other treatments such as biodegradation and oxidation. PAHs would be released from the sludge surface because of the effects of surfactants (Zheng et al., 2007). Then, PAHs in the aqueous phase easily can be removed via biodegradation and oxidation. In addition, surfactants can join polyelectrolytes to form polyelectrolyte-surfactant complexes, which favour removal of dyes (Choi et al., 2008). Moreover, heavy metals can desorb from sludge in the presence of surfactants. Micelles formed by surfactant molecules can bind with heavy metals, thus inhibiting the formation of metal deposits (Yuan and Weng, 2006). Recent studies have made considerable breakthroughs in the application of surfactants in WAS treatment. However, a comprehensive review of the advances in surfactant-assisted WAS treatment does not exist. Hence, this review summarizes the current application of surfactants in sludge treatment. It systematically clarifies the possible mechanisms of surfactants in sludge dewatering, anaerobic digestion, and removal of organic pollutants and heavy metals. In addition, the probable focus of future research in sludge treatment using surfactants is presented. This review provides a theoretical basis and direction for the application of surfactants in sludge treatment. 2. Surfactants in waste-activated sludge dewatering 2.1. Surfactants in traditional waste-activated sludge dewatering There is an urgent need to handle the tremendous amount of WAS generated from wastewater treatment activities (Mahmoud et al., 2006; Neyens et al., 2004). However, the disposal of sludge remains a crucial technical challenge owing to the high Wc of sludge (Zhang et al., 2016, 2015). Dewatering is an efficient method to reduce the cost of downstream treatment as it reduces Wc and sludge volume (Fu et al., 2009; Shi et al., 2015). Based on previous research, reducing bound water is the main method for improving sludge dewatering (Li et al., 2016; Qi et al., 2011). However, traditional mechanical dewatering treatment cannot effectively remove bound water in sludge flocs and decrease Wc of sludge (Jin et al., 2004; Novak et al., 2003). Surfactants can be used to enhance the dewaterability of sludge. In sludge dewatering treatment, after the addition of surfactants, the negative charge on the sludge surface can be neutralized by surfactants. Consequently, the binding between the LB-EPSs and TB-EPSs of sludge


flocs is weakened, promoting substantial dissolution of EPSs (mainly the proteins and polysaccharides) in sludge. Then, bound water combined with EPSs released from the sludge floc surface enter the sludge liquid phase (Hong et al., 2015a). Moreover, LB-EPSs and TB-EPSs are turned into soluble EPSs (S-EPSs) because of their high hydration ability. The interstitial water entrapped inside the sludge flocs and the water held inside the sludge are also released because of the destruction of the floc structure. All, these factors jointly contribute to the release of bound water, further improving sludge dewaterability (Fu et al., 2009). The release of EPSs caused by surfactants is illustrated in Fig. 1 (Wang et al., 2014b). The adsorption of surfactants on the sludge surface is a vital step in sludge dewatering. García et al. (2004) investigated the sorption of quaternary ammonium-based surfactants on sludge. A positively charged nitrogen atom exists in a molecule of quaternary ammonium-based surfactant. Strong electrostatic attraction develops between the surfactant molecules and all types of negatively charged solids. Hence, surfactants adsorb on sludge and play a role in dewatering (García et al., 2004). The influence of alkyl chain length and water hardness also has been evaluated according to the physical-chemical properties of sludge. As the alkyl chain length in the cationic surfactant molecule increases, the adsorption of surfactants on sludge increases as well. Water hardness can reduce the critical micelle concentration value of the alkyl benzyl dimethyl ammonium homologues and the extent of their sorption to sludge (García et al., 2004; Garcia et al., 2006b). According to the research on the sorption of linear alkylbenzene sulfonates (LAS) on sludge particles, water hardness also influences the sorption (García et al., 2002). Specific resistance to filtration (SRF), capillary suction time (CST), and Wc are commonly used to estimate sludge dewaterability. Wang et al. (2014a) explored the influence of the cationic surfactants dodecyl trimethyl ammonium bromide (DTAB) and cetyl trimethyl ammonium bromide (CTAB) on sludge dewatering. The destruction of sludge flocs and the release of bound water occurred owing to the effects of the surfactants. For DTAB, Wc of the filter cake decreased from 80.5% to 76.7%, whereas for CTAB, it decreased from 80.5% to 73.8% (Fig. S1). The final Wc of the sludge cake after CTAB treatment was 65.0 ± 0.4%. Nevertheless, it is doubtful whether the SRF reduction was in accordance with the decrease of Wc. The relationship between SRF and Wc thus requires further research. The surfactant dodecyl dimethyl benzyl ammonium chloride (DDBAC) also has the beneficial effect of releasing bound water from sludge (Hong et al., 2015a; Wang et al., 2014b). When the DDBAC dosage was 75 mg/g, Wc of dewatered sludge dropped to 66.61%. The decreasing trend in Wc reduced when the dosage of DDBAC exceeded 75 mg/g. In the end, when the dosage of DDBAC was 150 mg/g, Wc reached its minimum value (62.95%) (Huang et al., 2015).

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Some surfactants play the opposite role in sludge dewatering, as reflected in research on the anionic surfactant sodium dodecyl sulfate (SDS) by Wang et al. (2014b). SDS increased both the negative charge and repulsive forces between the released EPSs and sludge flocs. The electrostatic repulsion and the hydrophobic interactions between SDS and EPSs boosted the extended structure of the EPSs. Finally, the addition of SDS led to an increase of water retained in sludge and the deterioration of sludge dewaterability. When the SDS dosage increased from 0.104 to 2.084 g/L, the SRF and CST of sludge increased significantly. When the dose of SDS was 2.084 g/L, the increases in SRF and CST were 528.4% and 410.9%, respectively. These changes of SRF and CST indicated the sludge dewatering performance had worsened (Wang et al., 2014b). 2.2. Combined treatment for dewatering waste-activated sludge Flocculants are widely used to improve the sludge dewatering performance during physical and chemical conditioning. Flocculants can overcome the repulsion between particles and increase the particle size of sludge, leading to improvement in the stabilization and dewatering capacity of sludge (Cheng et al., 2012). Therefore, the effects of flocculants combined with surfactants in sludge dewatering have been investigated. Sun et al. (2014) found that improvement in sludge dewatering was caused by a reduction of EPSs and an increase of floc size. Both surfactants and flocculants can reduce negative charges on the sludge surface and the repulsion between sludge particles. Therefore, the release of EPSs and bound water from sludge into aqueous phase occurred easily. Meanwhile, the sludge aggregates became easily packable and the stability of sludge was improved. The role of flocculants in sludge flocculation is enhanced simultaneously by charge neutralization, absorption bridging, and the mechanism of the electrostatic path (Zhu et al., 2012, 2011). The dewaterability of sludge ultimately improved (Sun et al., 2014). When the dosage of cationic polyacrylamide (CPAM) 1 and CPAM2 was 40 mg/L, SRF was 6.43 × 1012 and 5.88 × 1012 m/kg, respectively; after adding CTAB, it reduced to 5.93 × 1012 and 5.50 × 1012 m/kg, respectively. Thus, SRF reduction reached 15.1% (CPAM1 + CTAB) and 16.8% (CPAM2 + CTAB). Meanwhile, the sludge sedimentation rate and dry solid content were improved by about 9.2–15.1% and 8–21.2%, respectively. Surfactants combined with flocculants have a positive effect on sludge dewatering (Sun et al., 2014). The effects of surfactants and acids/alkalis on sludge conditioning also have been investigated for adjusting the permeability of microorganisms in sludge (Sun et al., 2014). Sludge flocs disintegrate under acidic/alkaline conditions. Then, the non-dissolvable EPSs are peeled off and converted into liquid phase. When combined with acid/alkali treatment, surfactants can enhance the dissociation of sludge floc

Fig. 1. Conceptual illustration of sludge dewatering process induced by surfactants (Wang et al., 2014a). Copyright 2014 Elsevier.

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structure, further promoting the dissolution of EPSs and the release of bound water in EPSs via solubilization, hydrolysis, and dispersion. After treatment using surfactants and acids/alkalis, the dewaterability of sludge can improve significantly. Hong et al. (2015a) explored the impact of an acid/alkali and the surfactant DDBAC on sludge dewatering. Sludge dewaterability improved after pre-treatment with DDBAC and a strong acid/alkali. Under the effect of DDBAC, the Wc and SRF reductions were 64.8% and 57.8%, respectively. When the pH changed to 4.84, Wc decreased further to 58.1%, marking a significantly change (Hong et al., 2015a). Chen et al. (2001) also found that the dewaterability of sludge improved when the pH was 2.5. When sludge was pre-treated with acid or bovine serum albumin (BSA) prior to conventional conditioners, Wc dropped about 3–5%. Meanwhile, Wc of sludge decreased about 7–11% with combined treatment of an acid and BSA (Chen et al., 2001). Fenton's reagent combined with surfactants also works well in improving sludge dewatering. Strong oxidizer hydroxyl radicals (•OH) generated in a Fenton system can oxidize sludge flocs and decompose organics to release bound water. Then, EPSs and bound water can be released easily via the hydrolysis of surfactants (Hong et al., 2016). The improvement of dewaterability is due to the effects of both surfactants and Fenton's reagent. Hong et al. (2016) investigated the effects of Fenton's reagent and DDBAC in sludge dewatering. After treatment with Fenton's reagent, Wc and CST of sludge decreased to 63.36% and 28.7 s, respectively. After the addition of DDBAC, Wc and CST of sludge decreased further to 57.17% and 17.2 s, respectively. Therefore, Fenton-DDBAC treatment can improve the dewaterability of sludge significantly (Hong et al., 2016). Electrolysis is regarded as a useful method for enhancing sludge dewaterability because of its negligible environmental impact compared with other methods (Yuan et al., 2011a, 2011b). During electrolysis, floc structure and the cell membranes of microorganisms of sludge can be destroyed, leading to the release of EPSs (Keiding and Nielsen, 1997). After the addition of surfactants, more EPSs are released from the sludge surface and water trapped in sludge flocs can be released. Hence, the synergistic effects of electrolysis and surfactants have improved sludge dewatering. Yuan et al. (2011a) investigated the effects of electrolysis combined with surfactants in sludge dewatering. The results indicated that octylphenoxy polyethoxyethanol (Triton X-100) and SDS had negative effects on sludge dewatering both with and without electrolysis, whereas CTAB had a positive effect (Fig. S2). With Triton X-100 and SDS treatment, SRF and CST showed increasing trends. With CTAB treatment, SRF and CST both obviously decreased.

Furthermore, the dosage of CTAB had significant impact on dewatering performance in combined conditionings. The optimum results were achieved when the dosage was 2000 mg/L, with the reductions of SRF and CST reaching 224.3% and 298.8%, respectively, when combined with electrolysis. Contrarily, the reductions of SRF and CST were only 18.5% and 31.7%, respectively, when using only CTAB (Yuan et al., 2011a). The effects of the most commonly used surfactants in sludge dewatering are summarized in Table 1. 3. Surfactants in anaerobic digestion Anaerobic digestion is a promising approach for utilizing carbon sources in WAS and producing valuable energy sources such as methane (Appels et al., 2008; Mottet et al., 2009; Tomei and Carozza, 2015). Generally, the yield of methane is decided by the production of VFAs, which are significant intermediate products in anaerobic digestion and serve as a substrate for methane production (Elefsiniotis et al., 2004; Lemos et al., 2006; Yuan et al., 2014; Zhang et al., 2009). In the hydrolysis stage, enzymes can utilize the OM in sludge to produce lowmolecular-weight intermediates (Nybroe et al., 1992). However, the degradation of intracellular organics is hindered because the electrostatic interaction between enzymes and EPSs causes the formation of EPS-enzyme complexes, which trap enzymes in sludge. Therefore, the enzymatic reaction on sludge decreases because of the formation of complexes of extracellular enzymes and EPSs (Guo and Xu, 2011). Consequently, the rate of anaerobic digestion is restricted by the incomplete hydrolysis of EPSs and limited microbial biomass, leading to a poor yield of VFAs (Eastman, 1981; Tamilarasan et al., 2017; Yang et al., 2013). The hydrolysis of particulate OM into soluble substances is believed to be the main factor influencing the rate of anaerobic digestion (Luo et al., 2012). To improve the yield of VFAs and methane, surfactant assisted treatments are regarded as an alternative strategy for anaerobic digestion (Ebenezer et al., 2015; Jiang et al., 2007b; Kavitha et al., 2016b). Owing to their characteristics of high surface activity and solubilization, surfactants can change the tension and properties of sludge and promote the dissociation of sludge flocs. Hence, some TB-EPSs and LBEPSs start to transform into S-EPSs, causing a large amount of proteins and carbohydrates to be released from sludge into aqueous phase. Meanwhile, surfactants can denature proteins and carbohydrates in sludge flocs via tertiary structure unfolding. This is possibly caused by hydrophobic interactions favoured by thermodynamic effects and electrostatic interactions between amino acid residues. The utilization of

Table 1 The effects of some surfactants in sludge dewatering. Methods

DTAB CTAB DDBAC CPAM1 CTAB + CPAM1 CPAM2 CTAB + CPAM2 DDBAC DDBAC+ acid/alkali Acid BSA + acid Triton X-100 Triton X-100 + electrolysis SDS SDS + electrolysis CTAB CTAB + electrolysis

Surfactant dosage

1.042 g/L 1.042 g/L 150 mg/g − 0.8 mg/L − 0.8 mg/L 75 mg/g 75 mg/g − 0.1 g 3000 mg/g 3000 mg/g 3000 mg/g 3000 mg/g 3000 mg/g 3000 mg/g

Raw sludge Wc (%)

75.4 ± 2.6 75.4 ± 2.6 95 98.2–99.6 98.2–99.6 98.2–99.6 98.2–99.6 95–99 95–99 99.5 99.5 99.03–99.09 99.03–99.09 99.03–99.09 99.03–99.09 99.03–99.09 99.03–99.09

Dewatering parameters (%)

Ref

Wc of sludge

SRF reduction

CST reduction

68.9 ± 0.1 65.0 ± 0.4 63.0 − − − − 64.8 58.1 76.08 73.99 − − − − − −

42.8 31.3 − 6.9 15.1a 7.8 16.8a 57.8 57.8 − − −166.5 −386.2 −181.1 −89.6 18.5 224.3

− − − − − − − − − − − −67.7 −175.1 −367.3 −440.7 31.7 298.8

(Wang et al., 2014a) (Wang et al., 2014a) (Huang et al., 2015) (Sun et al., 2014) (Sun et al., 2014) (Sun et al., 2014) (Sun et al., 2014) (Hong et al., 2015b) (Hong et al., 2015b) (Chen et al., 2001) (Chen et al., 2001) (Yuan et al., 2011a) (Yuan et al., 2011a) (Yuan et al., 2011a) (Yuan et al., 2011a) (Yuan et al., 2011a) (Yuan et al., 2011a)

The full names of the abbreviations in the table are listed as follows: dodecyl trimethyl ammonium bromide (DTAB), cetyl trimethyl ammonium bromide (CTAB), dodecyl dimethyl benzyl ammonium chloride (DDBAC), cationic polyacrylamide (CPAM), bovine serum albumin (BSA), octylphenoxy polyethoxy (Triton X-100), and sodium dodecyl sulfate (SDS). a The value in this table is estimated from author's figures.


proteins and carbohydrates becomes simpler with the help of surfactants (Bhuyan, 2010). In addition, the solubility of surfactants might be vital for liberating trapped enzymes (on the cell surface and in the floc matrix) (Luo et al., 2011; Mayer et al., 1999). The increase of soluble organics and release of enzymes boost the hydrolysis rate and the possibility of enzymes hydrolysing organic compounds. Moreover, surfactants, acting as enzyme modulator molecules, increase enzyme activity at low dosage but have a contrary effect at high dosage. More protease and amylase activity can be obtained in the presence of surfactants. Regarding the release of EPSs induced by surfactants, bacteria can utilize these dissolved substances to produce VFAs and methane efficiently (Kavitha et al., 2014). Bacteria also can utilize surfactants as a carbon source (Carvalho et al., 2004). Kavitha et al. (2016a) investigated the effects of SDS on anaerobic digestion of sludge. With SDS treatment, methanogenic microbes could more easily access substrates, and methane yield was enhanced at the same time. When sludge retention time was 15 days, the methane yield reached its maximum (50 mL/g VS) (Kavitha et al., 2016a). In the short-term, the presence of SDS promoted the hydrolytic activities of enzymes. However, with an increase of time, the activities decreased rapidly. The productivity of VFAs improved with higher SDS dosage (Jiang et al., 2007a). Longer fermentation time was also required to improve VFA yield. In the end, the production of VFAs reached its maximum (460 mg/L) at 25 days (Kavitha et al., 2016a). Other surfactants have also been used in anaerobic digestion. After adding the surfactant sodium dodecylbenzene sulfonate (SDBS) for treatment, the concentrations of soluble proteins and carbohydrates increased, providing more substrates for acidification to produce VFAs (Ji et al., 2010). The bio-conversion of hydrolysis products such as amino acids and monosaccharides to VFAs was enhanced significantly. In anaerobic fermentation, the improved production of total VFAs (TVFAs) was caused mainly by biological effects, rather than by chemical effects and the degradation of SDBS itself. When sludge retention time (SRT) was 12 days, the VFA production was 2056 mg COD/L (Jiang et al., 2007b). The addition of polyoxyethylene sorbitan monooleate (Tween-80) and polyethylene glycol (PEG 6000) substantially increased cumulative hydrogen production and hydrogen yield (Elsamadony et al., 2015). When using surfactant LAS in anaerobic digestion, a negligible microbial transformation of the LAS molecule was observed, which may reduce the extent of methane production. When the concentration of LAS was 10 mg COD/L, the methane yield reached a maximum of 180 mg COD/L (Garcia et al., 2006a). The methane production improved significantly by using dioctyl sodium sulphosuccinate (DOSS). When the dosage of DOSS was 0.009 g/g SS, the methane production reached 0.225 g COD/g COD on the 15th day (Ushani et al., 2017).

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Apart from chemical surfactants, biosurfactants have attracted more attention owing to their lower toxicity, better environmental compatibility, biodegradability, and availability for a wide range of pH values and temperatures (Luo et al., 2013; Mulligan, 2005). Rhamnolipid had positive effects on sludge hydrolysis and acidification (Wu et al., 2014; Zhou et al., 2015). The role of rhamnolipid in sludge hydrolysis and acidification is illustrated in Fig. 2. Compared with chemical synthetic surfactants, rhamnolipid had a more positive effect on the solubilization of particulate organics from sludge flocs (Zhou et al., 2015) (Fig. S3). After the addition of rhamnolipid, SDS, and SDBS, the maximum VFA concentrations were 5844 ± 77, 5036 ± 11, and 1611 ± 61 mg COD/L, respectively. The concentration of VFAs obtained with rhamnolipid was up to 3.63 times higher than that obtained with SDBS and 1.16 times that of SDS. Rhamnolipid enhanced the efficiency of hydrolysis, which was beneficial to functional microorganisms for further acidification during sludge fermentation (Zhou et al., 2015). In anaerobic digestion, Pseudomonas aeruginosa oxidized simple carbohydrates to produce rhamnolipid (Van Rijn and Barak, 1996) and caused the generation of in-situ rhamnolipid (Zhou et al., 2013). The biosurfactant alkyl polyglycosides (APG) also enhanced sludge hydrolysis and VFA accumulation (Xu et al., 2015). The effects of the most commonly used surfactants in anaerobic digestion are concluded in Table 2. 4. Surfactants in organic contaminant decontamination As a final product of the wastewater treatment process, WAS contains many kinds of organic contaminants. PAHs and dyes are two main organic pollutants in sludge and which are non-degradable and toxic. Surfactants can be used to remove these pollutants owing to their solubilization. In addition, surfactants can cooperate with other methods in the removal of organic pollutants. PAHs can be effectively removed via anaerobic digestion or oxidation treatment (Fernando Bautista et al., 2009; Niepceron et al., 2010). The application of surfactants in these disposal methods can help to promote the removal of PAHs. In anaerobic digestion, surfactants are used to improve the dissolution of substrates via decreasing interfacial tension between substrates and sludge (Mier et al., 1995). The diffusion of surfactant molecules to the cell surface or enzyme sites controls the biodegradation of PAHs. Because of their high surface activity and dissolution, surfactants can promote the dissociation of sludge, as some PAHs could dissolve in aqueous phase from the sludge surface. Other PAHs can be encapsulated by micelles formed by surfactants molecules. PAHs dissolved in aqueous phase and micelle phase can be degraded rapidly by bacteria and other microbes (Bernal-Martinez et al., 2007; Sartoros et al., 2005; Trably and Delgenes, 2003; Trapido et al., 1995).

Fig. 2. A schematic presentation of proposed mode of rhamnolipid action on hydrolysis and acidification.

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Table 2 The effects of surfactants in anaerobic digestion. Surfactant

Ionic type

Dosage

VFAs production

SRT (d)

Methane yield

Ref

SDS SDS SDS SDS SDS SDBS LAS LAS SDBS SDBS SDBS DOSS Rhamnolipid Rhamnolipid APG AE

Anionic Anionic Anionic Anionic Anionic Anionic Anionic Anionic Anionic Anionic Anionic Anionic Anionic Anionic Nonionic Nonionic

0.04 g/g SS 0.04 g/g TSS 0.10 g/g DS 0.10 g/g 0.03 g/g SS 0.02 g/g 10 mg COD/L 75.40 ± 7.50 mg/L 0.02 g/g 0.04 g/g TSS 0.02 g/g 0.009 g/g SS 0.30 g/g DS 0.04 g/g TSS 0.20 g/g TSS 123.40 ± 34 mg/L

460 mg/L 5036 ± 11 mg COD/L 1457.12 mg COD/L 2243.04 mg COD/L _ 2056 mg COD/L _ _ 173.90 ± 6.60 mg COD/gVSS 1611 ± 61 mg COD/L 2188.93 mg COD/L _ 0.313 g COD/g 5844 ± 77 mg COD/L 2222 mg/L _

15 3 7 6 20 12 _ 28 6 3 6 15 9 3 2 28

0.05 L/g VS _ _ _ 2.52 L/g VS _ 180 mg COD/L 1238.1 μmol _ _ _ 0.009 g/g SS _ _ _ 1228.40 μmol

(Kavitha et al., 2016a) (Zhou et al., 2015) (Luo et al., 2011) (Jiang et al., 2007a) (Kavitha et al., 2016a) (Chen et al., 2013) (Garcia et al., 2006c) (Motteran et al., 2014) (Ji et al., 2010) (Zhou et al., 2015) (Jiang et al., 2007b) (Ushani et al., 2017) (Luo et al., 2013) (Zhou et al., 2015) (Xu et al., 2015) (Motteran et al., 2014)

The full names of the abbreviations in the table are listed as follows: sodium dodecyl sulfate (SDS), sodium dodecylbenzene sulfonate (SDBS), linear alkylbenzene sulfonates (LAS), dioctyl sodium sulphosuccinate (DOSS), alkyl polyglycosides (APG), and linear alkylethoxylate (AE).

Because surfactants increase the biodegradability of PAHs, the removal rate of PAHs during sludge anaerobic digestion is enhanced (BernalMartinez et al., 2007). Naphthalene is one of the main PAHs; Jegan et al. (2010) investigated the treatment of naphthalene by Micrococcus sp. and surfactants. Micrococcus sp. showed high naphthalene degradation potential. Triton X-100 and Tween-80 were used to enhance the bioavailability of naphthalene to microbes. Both Triton X-100 and Tween-80 boosted the performance of Micrococcus sp. in biodegrading naphthalene. When the naphthalene concentration increased from 500 to 5000 mg/L, the biodegradation of naphthalene ranged from 96.2 to 30.3% in the presence of Triton X-100 and decreased from 97.2 to 29.8% when Tween-80 was used (Jegan et al., 2010). In a Fenton reaction, surfactants can improve the effect of removing PAHs. In this combined treatment, PAHs are desorbed from the sludge surface and dissolved in aqueous and micelle phases through the action of surfactants. Then, •OH generated by a Fenton reaction oxidize and remove PAHs effectively. The mechanism of surfactants cooperating with Fenton's reagent is illustrated in Fig. 3. The surfactant polyoxyethylene lauryl ether (Brij-35) was more suitable than SDS, as it consumed less •OH. With Brij-35, the removal rate of benzo[a]pyrene reached 85.7% (Flotron et al., 2003). It was easier to remove PAHs that had PAHs had lower molecular weight (Zheng et al., 2007). In the treatment with Fenton's reagent and surfactants, it is noteworthy that the competition

between the dissolved OM and PAH reabsorption on the matrix limited the oxidation of PAH (Flotron et al., 2003). However, some surfactants cannot cooperate with oxidation treatment. In the METIX-AC process (Fenton-like reaction), the addition of surfactants may play the opposite role. After the addition of the nonionic surfactant Tween-80, the removal rate of PAHs by METIX-AC and Tween-80 (12 ± 5.2%) was less than that of METIX-AC (30 ± 7.7%). This may due to the double bond in the lipid chain of Tween-80 that is easily cleaved by Fenton-like reagents. Because the isolated double bond in Tween-80 was more easily oxidized than the conjugated π bond of PAHs, Tween-80 would lose its function. Similar surfactants with saturated lipophilic side chains may be a possible solution to avoid this defect and should be studied in future works (Zheng et al., 2007). The effects of surfactants in sludge treatment via oxidation and biodegradation have also been studied. Bernal-Martínez et al. (2005) investigated the role of surfactants in the removal of PAHs by combining ozonation with anaerobic digestion. With the action of oxidation and dissolution by ozone and surfactants, the biodegradation of PAHs improved significantly. The PAH removal rate increased to 61% via ozonation pre-treatment. Additional enhancement (up to 81%) of the PAH removal rate was achieved by adding the surfactant tyloxapol. In the presence of the surfactants tergitol and Brij-35, the removal rates of PAHs were 79% and 77%, respectively (Bernal-Martínez et al., 2005).

Fig. 3. The Mechanism of removing PAH by Fenton's reagent and surfactants in sludge.


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Table 3 Surfactants in some treatments of removing organic contaminants. Organic contaminants

Method

Surfactant

Dosage of surfactant

Removal rate (%)

Ref

Naphthalene Naphthalene PAHs PAHs PAHs PAHs PAHs PAHs PAHs Dye green

Biodegradation and surfactant treatment Biodegradation and surfactant treatment Fenton reaction and surfactant treatment Fenton reaction and surfactant treatment METIX-AC and surfactant treatment Mesophilic aerobic digestion and surfactant treatment Ozonation, surfactant treatment and biodegradation Ozonation, surfactant treatment with biodegradation Ozonation, surfactant treatment with biodegradation Polyelectrolyte and surfactant treatment

Triton X-100 Tween-80 Brij-35 SDS Tween-80 Tween-80 Tyloxapol Tergitol Brij-35 Quartolan

3 mM 1 mM 5.1–5 M 3 mM 1.5 g/L 0.5 g/L 1 g/L 1 g/L 1 g/L 0.003 mol/L

96.2–30.3 97.2–29.8 85.7 _ 12 ± 5.2 94 ± 0.5 81 79 77 99

(Jegan et al., 2010) (Jegan et al., 2010) (Flotron et al., 2003) (Flotron et al., 2003) (Zheng et al., 2007) (Zheng et al., 2007) (Bernal-Martínez et al., 2005) (Bernal-Martínez et al., 2005) (Bernal-Martínez et al., 2005) (Petzold and Schwarz, 2006)

The full names of the abbreviations in the table are listed as follows: polycyclic aromatic hydrocarbons (PAHs), octylphenoxy polyethoxy (Triton X-100), polyoxyethylene sorbitan monooleate(Tween-80), and polyoxyethylene lauryl ether (Brij-35).

However, tergitol showed an antagonistic effect in another study by Bernal-Martinez et al. (2007). Tergitol restricted the activity of an anaerobic ecosystem and increased PAH bioaccessibility (Sartoros et al., 2005). Dyes are widely applied in many industries, such as textiles, paper printing, leather, colour photography, and cosmetics (Jiang et al., 2017a; Leng et al., 2015b, 2015c; Wu et al., 2017a; Yuan et al., 2015b). However, dyes are difficult to remove, and that which remains in sludge can cause tremendous risks to the environment and humans (Akkaya et al., 2007; Eren and Acar, 2006; Özcan et al., 2006). Polyelectrolytes and polyelectrolyte-surfactant complexes can effectively remove dyes in sludge (Petzold and Schwarz, 2006). When a linear polyelectrolyte is present, the formation of water-insoluble three-component dye complexes can be boosted. Polyelectrolytes and polyelectrolyte-surfactant complexes exhibited high absorption to dyes owing to the triple component formation (Zemaitaitiene et al., 2003). The removal of dyes improves via the hydrophobic interaction between dyes and complexes. The charges of the complexes were also a factor. The best results were obtained with neutral complexes. The complexes had a very wide flocculation window, which could bind the dye particles effectively, because of its size and structure. The final removal rate of dye green (mixture of two commercial dyes) reached 99% (Petzold and Schwarz, 2006). The effects of the most commonly used surfactants in removing PAHs and dyes are summarized in Table 3. 5. Surfactants in heavy metal removal Heavy metals are stubborn environmental contaminants that are harmful to environmental and human health (Davidson et al., 1994; Jiang et al., 2017b; Wang et al., 2014c; Xu et al., 2006). There is an urgent need to remove heavy metals in WAS, and using surfactants in sludge treatment for such is feasible and effective. Heavy metals can be leached from sludge via treatment with surfactants. Surfactants cooperate with bioleaching, and the electrokinetic (EK) process also is used widely, as it

can remove heavy metals from sludge effectively (Kosobucki et al., 2008). Because of their self-assembly and binding abilities, surfactants can enhance the desorption of heavy metals from sludge. The mechanism (Fig. 4) of leaching heavy metal ions from the sludge surface with the aid of surfactants is described as follows (Kuczajowska-Zadrożna et al., 2015): i) adsorption of surfactant molecules from dissociated micelles on sludge, ii) adsorption of surfactants at head-head and tail-tail position, and iii) aggregation of chemimicelles and desorption of metalsurfactant complexes. In the process of removing heavy metals, saponin, tannic acid, and rhamnolipids showed great capability in removing heavy metals from the sludge surface, whereas JBR 515 was relatively ineffective. With JBR 515, the removal rates of Cu, Zn, and Cd were 26%, 42%, and 25%, respectively. Among these biosurfactants, saponin was the best in removing heavy metals. The removal rates by saponin of Cu, Zn, and Cr were 95%, 96%, and 91%, respectively. The sequences of leaching a series of metals from sludge with the described washing agents were as follows (Kuczajowska-Zadrożna et al., 2015):

Surfactants can improve the removal of heavy metals via bioleaching. During the bioleaching process, acidification of sludge through ferrous iron oxidation by Acidithiobacillus ferrooxidans or sulfur oxidation by A. thiooxidans play an important role in metal extraction. Elemental sulfur (S0) has been used extensively as a substrate for the growth of A. thiooxidans. The sulfuric acid produced by A. thiooxidans during S0 oxidation helps remove heavy metals (Liu et al., 2012). In bioleaching, surfactants addition reduces the surface tension of S0, further improving the efficiency S0 oxidation by of A. thiooxidans. The effectiveness of Tween-80 also has been demonstrated (Xin et al., 2009; Zeng et al., 2012).

Fig. 4. Mechanism of metals leaching from sludge by surfactants.

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Table 4 The effects of some surfactants in the treatments of removing heavy metals. Methods

Surfactant leaching Surfactant leaching Surfactant leaching Bioleaching, Fenton oxidation and surfactant treatment Bioleaching, Fenton oxidation and surfactant treatment Bioleaching, Fenton oxidation and surfactant treatment Bioleaching, Fenton oxidation and surfactant treatment Bioleaching, Fenton oxidation and surfactant treatment EK process and surfactant treatment

Surfactants

Tannin acid JBR 515 JBR 425 _ SDBS Tween-20 Tween-60 HTAC SDS

Dosage of surfactant

0.02 m/V 0.02 m/V 0.02 m/V _ 4 g/L 4 g/L 4 g/L 4 g/L 0.024 m

Heavy metals removal (%)

Ref

Cu

Zn

Cd

Pb

Cr

Fe

Ni

25 26 15 62a 7 79 90 8 41

35 42 39 _ _ _ _ _ 67

21 25 25 50a 5a 52a 60 4a 40

_ _ _ 24a 4a 25a 30 4a 59

_ _ _ _ _ _ _ _ _

_ _ _ _ _ _ _ _ 60

_ _ _ _ _ _ _ _ 78

(Kuczajowska-Zadrożna et (Kuczajowska-Zadrożna et (Kuczajowska-Zadrożna et (Kuczajowska-Zadrożna et (Ren et al., 2014) (Ren et al., 2014) (Ren et al., 2014) (Ren et al., 2014) (Ren et al., 2014)

al., 2015) al., 2015) al., 2015) al., 2015)

The full names of the abbreviations in the table are listed as follows: sodium dodecylbenzene sulfonate (SDBS), polyoxyethylene sorbitan monolaurate (Tween-20), polyethylene glycol sorbitan monostearate (Tween-60), hexadecyl trimethyl ammonium chloride (HTAC), and sodium dodecyl sulfate (SDS). a The value in this table is estimated from author's figures.

Ren et al. (2014) investigated the effects of four surfactants in bioacidification cooperation with a Fenton reaction process. Surfactants promoted the process of bio-acidification by enhancing the dissolution of organics. After the addition of surfactants, the dissolution of S0 from the sludge surface improved significantly. Then, the oxidation of S0 by sulfur-oxidizing bacteria resulted in the solubilization of heavy metals. Moreover, heavy metals were transformed from binding state into free state and released from the sludge via Fenton reaction. After the addition of a Fenton reagent, the removal rates of Cu, Pb, and Cd were 62%, 24%, and 50%, respectively. However, in combined treatments, SDBS and the surfactant hexadecyl trimethyl ammonium chloride (HTAC) played opposite roles. The removal rates of heavy metals with SDBS (Cu: 7%, Pb: 4%, Cd: 5%) and HTAC (Cu: 8%, Pb: 4%, Cd: 4%) were very low. Only polyoxyethylene sorbitan monolaurate (Tween-20) and polyethylene glycol sorbitan monostearate (Tween-60) proved effective in this research. With respect to SDBS and HTAC, the removal of Cu had a sharp decrease compared with those of Tween-20 and Tween-60. The removals of Pb and Cd were not as efficient as that of Cu (Ren et al., 2014). The removal of Pb was connected to the pH, which needed time to reach. The lower removal rate of Cd was seen because the removal of Cd required positive oxidation-reduction potential values and acidification varied from pH 2 to 4. Tween-60 and Tween-20 were valid in removing Cu, Pb, and Cd, especially Tween-60. The removals of Cu, Pb, and Cd by Tween-60 were 90%, 30%, and 60%, respectively (Chen and Lin, 2009). In combined treatment with the EK process and surfactants, the hydrogen bonding and electrostatic forces between surfactants and metals enhanced the desorption of heavy metals from sludge. Because of the binding counterions on the surface of micelles, heavy metals in sludge changed from residual to sorbed and organic fractions in this treatment. Metal residuals caused by OH¯ were diminished by the binding characteristic of the surfactants. Electrically induced ion migration was the most important cause of the transport of heavy metals (Jafvert and Heath, 1991). In the EK process combined with SDS, the removal priority of the investigated metals from sludge was: Cu N Pb N Ni N Fe N Zn N Cr, which might be largely related to the ion mobility of the metals. For EK-SDS treatment, the removal efficiency of metals was in the range of 37–77%. The removal rates of Ni and Pb were significant, at 77% and 51%, respectively (Yuan and Weng, 2006). The effects of the most commonly used surfactants in removing heavy metals are summarized in Table 4. 6. Conclusion This review summarizes the application of surfactants in sludge treatment. The use of surfactant-based remediation can overcome the limitations of traditional methods such as low efficiency and lack of flexibility. In sludge dewatering, surfactants have the capability of dissolving EPSs and thus releasing bound water from sludge. In anaerobic

digestion, surfactants can convert EPSs to soluble organics, which can be used for enhancing acidification and methanogenesis. In addition, the surfactant micelles of surfactants can encapsulate hydrophobic organics interiorly and capture contaminants on the exterior surface, effectively favouring the removal of PAHs and heavy metals. However, the following deficiencies of surfactants treatment exist. i) The relationship between SRF reduction and dewaterability after the addition of surfactant needs to be clarified. ii) The mechanism of surfactants enhancing the biological effects of enzymes in hydrolysis needs to be studied. iii) The secondary pollution caused by surfactants needs to be determined urgently. Then, it needs to be eliminated. Because of the toxicity of synthetic surfactants, biosurfactants have been successfully employed in sludge treatments. Nevertheless, large-scale application of biosurfactants is still limited owing to the high costs and low yields. How to improve the production and decrease the cost of biosurfactants is worthy of investigation. Acknowledgments The authors gratefully acknowledge the financial support provided by the Foundation for Innovative Research Groups of the National Natural Science Foundation of China (No. 51521006), Key Project of National Nature Science Foundation of China (No. 71431006), and Key Research and Development Project of Hunan Province, China (No. 2016SK2015). Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.scitotenv.2017.07.189. References Akkaya, G., Uzun, İ., Güzel, F., 2007. Kinetics of the adsorption of reactive dyes by chitin. Dyes Pigments 73, 168–177. Appels, L., Baeyens, J., Degreve, J., Dewil, R., 2008. Principles and potential of the anaerobic digestion of waste-activated sludge. Prog. Energy Combust. Sci. 34, 755–781. Appels, L., Houtmeyers, S., Degreve, J., Van Impe, J., Dewil, R., 2013. Influence of microwave pre-treatment on sludge solubilization and pilot scale semi-continuous anaerobic digestion. Bioresour. Technol. 128, 598–603. Beneito-Cambra, M., Herrero-Martínez, J.M., Ramis-Ramos, G., 2013. Analytical methods for the characterization and determination of nonionic surfactants in cosmetics and environmental matrices. Anal. Methods 5, 341–354. Bernal-Martínez, A., Carrère, H., Patureau, D., Delgenès, J.P., 2005. Combining anaerobic digestion and ozonation to remove PAH from urban sludge. Process Biochem. 40, 3244–3250. Bernal-Martinez, A., Carrere, H., Patureau, D., Delgenes, J.P., 2007. Ozone pre-treatment as improver of PAH removal during anaerobic digestion of urban sludge. Chemosphere 68, 1013–1019. Bhuyan, A.K., 2010. On the mechanism of SDS-induced protein denaturation. Biopolymers 93, 186–199. Carvalho, G., Novais, J.M., Pinheiro, H.M., Vanrolleghem, P.A., 2004. Model development and application for surfactant biodegradation in an acclimatising activated sludge system. Chemosphere 54, 1495–1502.

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