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Insight into a new two-step approach of ozonation and chitosan conditioning for sludge deep-dewatering
Science of the Total Environment 697 (2019) 134032
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Science of the Total Environment journal homepage: www.elsevier.com/locate/scitotenv
Insight into a new two-step approach of ozonation and chitosan conditioning for sludge deep-dewatering Dongdong Ge a, Wenrui Zhang a, Chang Bian a, Haiping Yuan a, Nanwen Zhu a,b,⁎ a b
School of Environmental Science & Engineering, Shanghai Jiao Tong University, Shanghai 200240, China Shanghai Institute of Pollution Control and Ecological Security, Shanghai 200092, China
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
• A new two-step approach was proposed for sludge deep-dewatering. • CT neutralized and flocculated negative ozonated sludge colloids efficiently. • Heavy metals contents of conditioned sludge cake met A level of agricultural land. • Most heavy metals' chemical speciation altered to higher toxicity and bioavailability.
a r t i c l e
i n f o
Article history: Received 12 June 2019 Received in revised form 1 August 2019 Accepted 20 August 2019 Available online 22 August 2019 Editor: Deyi Hou Keywords: Sludge deep-dewatering Ozonation Chitosan (CT) Sludge properties Heavy metals
a b s t r a c t Sludge deep-dewatering, capable of reducing water content (Wc) of sludge cake under 60%, is one of the current hot topics in sludge treatment. In this study, a new environmentally friendly two-step approach of ozonation and chitosan (CT) conditioning was proposed and examined to be practicable for sludge deep-dewatering. With 60 mg/gTS ozone and 20 mg/gTS CT conditioning, sludge capillary suction time (CST) and Wc of the dewatered sludge cake decreased from 196.3 s and 84.7% of raw sludge to 15.8 s and 57.5%, respectively. Ozonation treatment could efficiently crack sludge flocs and cells, and release biopolymers, causing the decreases in viscosity, zeta potential and particle size. Subsequently, CT ameliorated the sludge dewaterability successfully by neutralizing negative charges and flocculating colloids to promote the spread of interstitial water. Furthermore, the contents of heavy metals (As, Cd, Cr, Ni, Pb and Zn) in conditioned sludge cake decreased obviously except Cu, but all detected heavy metals contents satisfied the A level of agricultural land (GB4284-2018). For chemical speciation of heavy metals, the proportions of the acid soluble/exchangeable state and the reductive state increased apparently, implying higher toxicity and bioavailability, except Pb. Hence, pretreatments were required to reduce the environmental risk of heavy metals in conditioned sludge cake prior to a further utilization. © 2019 Elsevier B.V. All rights reserved.
1. Introduction
⁎ Corresponding author at: School of Environmental Science & Engineering, Shanghai Jiao Tong University, Shanghai 200240, China. E-mail address: [email protected] (N. Zhu).
https://doi.org/10.1016/j.scitotenv.2019.134032 0048-9697/© 2019 Elsevier B.V. All rights reserved.
In China, the annual municipal waste activated sludge (WAS) production has exceeded 42 million tons and is still growing (Ge et al., 2019a). To reduce the sludge output of sewage treatment plant, researchers are beginning to looking for a deep-dewatering technique
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capable of reducing water content (Wc) of sludge cake under 60%. However, so far, due to the high WAS moisture of 95–99% and unsatisfactory dewaterability, the Wc of dewatered sludge cake after mechanical dewatering with conventional coagulant and/or flocculant still remains high at around 75–80% (Zhen et al., 2018). Hence, sludge conditioning becomes particularly important for further enhancing WAS dewaterability. A wide variety of conditioning techniques have been investigated for WAS dewatering such as thermal hydrolysis (Deng et al., 2019), microwave (Wojciechowska, 2005; Zhen et al., 2019), electrolysis (H. Yuan et al., 2011), ultrasound (Feng et al., 2009), bioleaching (Huo et al., 2014), and advanced oxidation processes (AOPs) (Liu et al., 2016; Lu et al., 2003; Zhen et al., 2018). Nevertheless, due to the highly hydrophilic colloid structure of sludge flocs and rigid sludge cell walls making sludge dehydration extremely hard, comprehensive disintegrations of sludge extracellular polymeric substances (EPS) and breakdowns of sludge cell walls are greatly imperative for the thorough release of intracellular and chemically bound water. Noted worthily, lots of the single technique such as electrolysis, ultrasonication, and bioleaching remain in decomposing sludge EPS to enhance sludge dewatering due to their mild degradation capacity and overtreatment will cause the dewaterability deterioration again. Differently, AOPs represented by Fenton and Fe2+/persulfate processes can quickly crack sludge flocs and cell walls to release intracellular substances and highly degrade sludge EPS to harvest efficient sludge deepdewatering via the strongly oxidative free radicals and re-flocculation effect (Zhen et al., 2018). However, Fenton reagents exerted an excellent role at the demanding operating condition of pH 3.0 (Lu et al., 2003); Fe2+/persulfate reagents of 1.5 mmol/gVSS Fe2+ and 1.2 mmol/gVSS persulfate could achieve the desired sludge dewaterability rapidly with 1 min (Zhen et al., 2012), but cost more. Moreover, the additive Fe2+ easily induces filter cloth clogging, pump wear, equipment life-shortening and sludge calorific value reduction in actual operation. Therefore, it is greatly necessary to seek a more feasible, environmentally friendly and economical approach for sludge deep-dewatering. Ozonation, as one promising technique for efficiently cracking sludge cells with no secondary toxic by-products and moderate operating cost, has been revealed to cause 60% sludge suspended solid reduction and the leakages of carbon, nitrogen and phosphorus from sludge
substrate into supernatant (Sang-Tian et al., 2009). Unfortunately, the physicochemical properties of ozonation sludge altered dramatically and sludge dewaterability deteriorated seriously (Zhang et al., 2016; Zhao et al., 2007). Thus, it is quite meaningful to consider adding a subsequent treating following sludge ozonation to reverse the major factors affecting sludge dewaterability for realizing a nontoxic and harmless dewatering process. Considering the solubilization of sludge biopolymers into small molecular organic matters with negative charges and the high cellular lysis efficiency causing the sludge particle size reduction after ozonation, we deduce that charge neutralization and flocculation are possibly favorable for further dewaterability amelioration of ozonation sludge. Chitosan (CT), as the only natural and renewable cationic biopolymer with the outstanding properties of non-toxicity, biocompatibility as well as biodegradability, has been extensively adopted in food processing, pharmaceutical application and sewage treatment (No and Meyers, 2000; Rinaudo, 2006; Zhang et al., 2010). It is an aminopolysaccharide with a linear and semi-rigid structure and efficaciously functions as the adsorbent and/or flocculant for capturing metal ions and polymers such as protein by active hydroxyl and amino groups (No and Meyers, 2000; Zemmouri et al., 2015), so CT has great potential to be the alternative replacement of synthetic organic flocculant (PAM), and the formed CT sludge can serve as additive in fertilizers and animal feed (Bratskaya et al., 2004). At present, there are a few reports with respect to the application of CT in sludge dewatering. However, the investigated contents only focused on the dewaterability effects (Cooper et al., 1986; Zemmouri et al., 2015), and lacked the research concerning sludge properties which were greatly instrumental in exploring dewatering mechanism by CT. Other researchers devoted to the synthesis of chitosan-based flocculant and emphasized the characteristics of the modified materials without the discussion regarding their roles on sludge properties (Huang et al., 2016; Wang et al., 2016). Hence, attention must be paid on not only the apparent dewaterability but also the underlying conditioning mechanism. In this study, a novel attempt of two-step approach associating sludge ozonation with CT conditioning was conducted to realize sludge deep-dewatering in an environmentally friendly manner, of which flow chart was shown in Fig. 1. The sludge dewaterability was characterized
Fig. 1. Flow chart of the new two-step approach of ozonation and CT conditioning for sludge deep-dewatering.
D. Ge et al. / Science of the Total Environment 697 (2019) 134032
by capillary suction time (CST) and Wc of dewatered sludge cake which was acquired by a laboratory filter press for being closer to the actual operating value. Sludge properties including viscosity, zeta potential, particle size, EPS (protein and polysaccharide) contents as well as the contents and chemical speciation of heavy metals (As, Cd, Cr, Cu, Ni, Pb and Zn) in dewatered sludge cake were especially monitored for a comprehensive insight into the mechanism of sludge deep-dewatering by this two-step method.
2. Materials and methods
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2.2. Experimental process Sludge sample of 200 mL fed into the reactor was subjected to ozonation and the ozone dosage was calculated as the quantity of inlet ozone quantity minus outflow ozone quantity. Various ozone dosages (20 mg/gTS, 40 mg/gTS, 60 mg/gTS, 80 mg/gTS and 100 mg/gTS) were added by adjusting reaction time. Then, the obtained ozonation sludge samples continually stirred overnight for completely consuming the residual ozone. 150 mL ozonation sludge was conditioned using chitosan solution on a magnetic stirring apparatus with the stirring speed of 350 rpm for 3 min and the sludge samples without ozonation treating and/or CT conditioning were set as control groups.
2.1. Materials 2.3. Analysis methods WAS was gained from the secondary settling tank of a municipal sewage treatment plant, which adopted an anaerobic-anoxic-oxic process in Shanghai, China. The collected sludge was then screened through 1.0 mm sieve to discard the gravel and hairs and subsequently centrifuged for thickening the WAS with about 3% total solid (TS). The characteristics of the prepared sludge were as follow: total solid (TS) of 31.2 ± 0.5 g/L, volatile solid (VS) of 18.9 ± 0.6 g/L, pH value of 7.24 ± 0.02, CST of 196.3 ± 3.1 s and Wc of dewatered sludge cake of (84.7 ± 0.1)% prior to dewatering experiments. Ozone was produced using an ozone generator with pure oxygen as gas source and bubbled into the ozonation reactor made of plexiglass through the micropore aeration plate at the bottom of the reactor. The inlet and outflow ozone concentrations were monitored by a digital ozone concentration detector, and the inlet ozone concentration was set as 35 mg/L and the inlet flow was 1 L/min. CT (deacetylation degree ≥95%, viscosity of 100–200 mPa·s) was purchased from Macklin Co. Ltd. The 1.5% (wt%) chitosan solution prepared with 5% acetic acid solution was employed to condition sludge.
2.3.1. Sludge dewaterability The CST value was detected using a 304B CST instrument (model, Triton, UK). The water content (Wc) of the dewatered sludge cake gained through a laboratory filter press with 100 mL sludge sample at a pressure of 0.6 MPa for 10 min was calculated according to the standard method (APHA, 1998). 2.3.2. EPS extraction and analysis Sludge EPS extraction was abided by the method described in Zhen's research (Zhen et al., 2013). Various extracted supernatant samples defined as slime EPS (S-EPS), loosely bound EPS (LB-EPS) and tightly bound EPS (TB-EPS) were further filtered through 0.45 μm microfiber membrane for protein and polysaccharide determinations. 2.3.3. Sludge properties TS and VS of sludge samples were determined according to the standard methods (APHA, 1998). Sludge viscosity was monitored by a rotary
Fig. 2. Variations of (a) sludge CST at various ozone dosages; variations of (b) sludge CST and (c) Wc of dewatered sludge cake with various ozone dosages and 20 mg/gTS conditioning; variations of (d) sludge CST and (e) Wc of dewatered sludge cake with 60 mg/gTS ozone treatment and CT conditioning at various CT dosages.
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Fig. 3. Variations of sludge viscosity with (a) various ozone dosages and (b) 60 mg/gTS ozone treatment and CT conditioning at various CT dosages.
viscosimeter (NDJ-8S). Sludge particle size distribution was analyzed by a Malvern UK Mastersizer 3000. The zeta potential was determined with a Beckman Coulter Delsa™ Nano instrument. The pH value was determined with a digital pH-meter (pHs-3C). EPS protein was determined by the Bradford method (Bradford, 1976), using bovine serum albumin (BSA) as the standard. EPS polysaccharide was determined by the anthrone-sulfuric acid method (Hassid and Abraham, 1957), using glucose as the standard. The concentrations of heavy metals (As, Cd, Cr, Cu, Ni, Pb and Zn) were determined using an Agilent 5110 ICP-OES, and the chemical speciation of heavy metals was analyzed using BCR sequential extraction method (X. Yuan et al., 2011). 3. Results and discussion 3.1. The role of ozonation and CT conditioning on sludge dewaterability CST is valuably indicative of WAS dewaterability, especially sludge filterability (Chen et al., 2001). As shown in Fig. 2(a), the raw sludge CST was 196.3 s. With the treatment of ozone ranging from 20 mg/gTS to 80 mg/gTS, sludge CST deteriorated from 259.5 s to 712.3 s pronouncedly, which implied that sludge filterability turned poor and the moisture trapped in sludge became harder to remove. In order to further enhance the dewaterability of ozonated sludge, 20 mg/gTS CT was fed in the sludge at various ozone dosages. As shown in Fig. 2(b), after step 2 of CT conditioning, sludge CST further ameliorated apparently, revealing that CT successfully altered partial sludge properties
impeding sludge filterability after ozonation. Therefore, CT was also suitable for the conditioning of ozonated sludge, not only traditional municipal and digested sludge reported previously (Wang et al., 2019; Zemmouri et al., 2015). However, the more ozone dosage added in step 1, the larger CST of CT-conditioned sludge became, suggesting that the sludge with a higher ozonated degree was more difficult to adjust. In practical operation, Wc of dewatered sludge cake, as the most intuitive and meaningful indicator, reflected the degree of sludge volume reduction (Ge et al., 2019b). As shown in Fig. 2(c), with the conditioning of various ozone dosage and 20 mg/gTS CT, Wc of dewatered sludge cake firstly decreased and then slightly increased, implying that CT effectually strengthened the separation of solid sludge and water liquid after sludge ozonation. Noted worthily, when the ozone dosage was 60 mg/gTS, the Wc of dewatered sludge cake could be reduced to 57.5% b60%, indicating that this two-step approach of ozonation and CT conditioning was a promising and powerful technique to accomplish sludge deep-dewatering. Furthermore, the effect of various CT dosages on sludge dewaterability was further investigated under 60 mg/g TS ozone. As shown in Fig. 2(d) and (e), sludge CST and Wc of dewatered sludge cake could be reduced to b50 s and 60% with the CT dosage N15 mg/gTS, while the further cuts on CST and Wc would be slight with the CT dosage beyond 20 mg/gTS. Here, the CT dosage introduced in ozonated sludge was slightly higher than the report CT dosage used in municipal and digested sludge as well as peat (Cooper et al., 1986; Wang et al., 2019; Zemmouri et al., 2015), primarily resulting from the special ozonation sludge properties, which would be discussed in later section.
Fig. 4. Variations of sludge zeta potential with (a) various ozone dosages and (b) 60 mg/gTS ozone treatment and CT conditioning at various CT dosages.
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3.2. The role of ozonation and CT conditioning on sludge properties 3.2.1. Variations of sludge viscosity with ozonation and CT conditioning Sludge viscosity, as a valuable factor influencing WAS dewatering, has been observed to be positively correlated with sludge dewaterability in the previous reports (Li and Yang, 2007; Zhen et al., 2012). Interestingly, as shown in Fig. 3(a), with the increased ozone dosage ranging from 0 to 80 mg/gTS in step 1, sludge viscosity declined constantly from 412.0 mPa·s to 57.5 mPa·s, but sludge dewaterability worsened distinctly as depicted in Fig. 2(a). Further, as shown in Fig. 3 (b), with the added CT dosage ranging from 15.0 mg/gTS to 22.5 mg/gTS in step 2, sludge viscosity gradually decreased and then rose slightly, implying that the helpful CT was within a certain dosage range and excessive CT (N20 mg/gTS) led to the increased sludge viscosity again. Noted worthily, sludge viscosity increased by at least 6.0 mPa·s corresponding to the CT dosage of 20 mg/gTS, whereas the ozonated sludge dewaterability improved from CST of 692.4 s to CST
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of 15.8 s significantly. The occurred negative correlations between sludge viscosity and sludge dewaterability in the two steps were not consistent with the mentioned report (Zhen et al., 2012). The reason was probably that sludge was subjected to ozone oxidation and organic conditioner, different from the Fe2+/persulfate process plus inorganic re-flocculation in Zhen's investigation (Zhen et al., 2018). Fe2+/persulfate could effectively break down the sludge cells and EPS by generated strongly powerful sulfate radicals and hydroxyl radicals, and the flocculation effect of iron ions further largely remove sludge EPS (Zhen et al., 2012, 2018). Thus, sludge viscosity reduced and WAS dewaterability enhanced dramatically in Fe2+/persulfate process (Zhen et al., 2012). However, during the ozone oxidation, large sludge granules were gradually oxidized into small particles, inducing the continual release of bound water and sludge biopolymers as well as the increase of sludge negative charge, which were concretely discussed in the later sections. Hence, sludge viscosity reduced to some extent, but more released and stabilized biopolymers afforded more binding sites for water in
Fig. 5. Variations of sludge particle size with (a) various ozone dosages and (b) 60 mg/gTS ozone treatment and CT conditioning at various CT dosages.
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the sludge matrix, and sludge dewaterability worsened. Moreover, CT, as an organic conditioner, could flocculate sludge biocolloid via adsorption, crosslinking and bridging (Huang et al., 2016), which was different from the function of neutralizing charges and compressing double electric layer of inorganic coagulant (Niu et al., 2013). Similarly, tannic acid, as another natural organic conditioner, was also observed to enhance sludge dewaterability but increase sludge viscosity to a certain extent (Ge et al., 2019b). 3.2.2. Variations of sludge zeta potential with ozonation and CT conditioning Zeta potential can reflect the electrically charged sludge colloid accurately and the elimination of negative or positive charges would be favorable for the enhancement of WAS dewaterability (Ge et al., 2019b; Jin et al., 2004). As shown in Fig. 4(a), the zeta potential of raw sludge was −16.81 mV. With the increased ozone dosage, the sludge zeta potential decreased obviously. As ozone dosage was 60 mg/gTS, sludge zeta potential dropped to −24.42 mV and then altered mildly. This was because the negatively charged organic matters were continually released from the internal sludge cells and flocs into the external space of sludge during ozonation, and the released biopolymers carrying identical electrical charges repelled with each other, which further blocked the flow of water and caused poor dewaterability (Liu et al., 2001; Zhang et al., 2009). But to a certain level, the oxidized rate balanced the released rate, sludge zeta potential changed slightly. Further, as shown in Fig. 4(b), with the increased CT, sludge zeta potential gradually tended to 0. Especially, when the CT dosage was 20 mg/gTS, sludge zeta potential reached to −1.59 mV which was close to neutrality. The
results implied that CT, as a cationic electrolyte, could remarkably neutralize negative charges of organic biopolymers, such as protein and polysaccharide, and the sludge biocolloid would gradually destabilize, agglomerate and settle down (Wang et al., 2016).
3.2.3. Variations of sludge particle size with ozonation and CT conditioning Sludge particle size is closely related to sludge dewaterability (Karr and Keinath, 1978; Shao et al., 2009). As shown in Fig. 5(a), sludge particle size decreased significantly via ozonation, and especially, with 60 mg/gTS ozone treatment, sludge D50 and D90 decreased from 54.9 μm and 138 μm of raw sludge to 46.2 μm and 120 μm, respectively, which revealed that ozone efficiently cracked and oxidized large sludge flocs into small particles (Zhao et al., 2007). Subsequently, through the CT conditioning, sludge particle enlarged pronouncedly with the increased CT dosage, as depicted in Fig. 5(b). It was distinctly observed that, as the CT dosage was 20 mg/gTS, sludge D50 and D90 increased by 20.3 μm and 135 μm, respectively. However, in step 1, ozonation disruption would increase sludge surface area and expose more hydrophilic groups, strengthening the affinity of sludge colloids to water and further deteriorating sludge dewatering (Zhang et al., 2016; Zhao et al., 2007). In step 2, CT conditioning efficaciously flocculated the released colloids and enlarged ozonated sludge particle size, promoting the compression of aggregates and the spread of trapped interstitial water, which was consistent with the report (Cooper et al., 1986). Interestingly, it was clearly observed that a large number of aggregated flocs were formed, and water gradually spilled out from the agglomerations during CT conditioning experiments.
Fig. 6. Variations of (a) EPS protein and (b) EPS polysaccharide with various ozone dosages; variations of (c) EPS protein and (d) EPS polysaccharide with 60 mg/gTS ozone treatment and CT conditioning at various CT dosages.
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3.2.4. Variations of EPS contents with ozonation and CT conditioning Sludge EPS is secreted by the bacteria embedded in sludge flocs and protected their living surroundings from the exterior environment (Zhou et al., 2015). It mainly consists of protein, polysaccharide, lipid and nucleic acid, etc. (Frølund et al., 1996). Among the biopolymers, protein and polysaccharide account for 70–80% of the extracellular organic carbon (Neyens et al., 2004), and especially, EPS protein has been reported as the major factor governing WAS dewaterability (Shao et al., 2009; Wang et al., 2014; Wu et al., 2017). As depicted in Fig. 6(a) and (b), the biopolymers of sludge could be pronouncedly released into EPS by ozonation destroying sludge flocs and cell walls. At low ozone dosage (40 mg/gTS), S-EPS protein increased from 4.97 mg/L (raw sludge) to 36.03 mg/L as a result of the migration of protein from sludge inside to outside and sludge dewaterability worsened dramatically. With the ozone dosage over 40 mg/gTS, all of the protein contents in S-EPS, LB-EPS and TB-EPS decreased gradually, suggesting that protein degradation rate was higher than protein solubilization rate in the three EPS stratifications. Especially, with the ozone dosage of 60 mg/gTS, the proteins in sludge S-EPS, LB-EPS and TB-EPS altered from 4.97 mg/L, 23.11 mg/L and 28.61 mg/L of raw sludge to 10.20 mg/L, 1.49 mg/L and 2.27 mg/L, respectively, revealing that ozone could efficaciously dissolve, transfer, degrade and even mineralize sludge proteins. The result was different from the previous report showing the continual increase of EPS protein in ozonation treatment (Zhang et al., 2016), which was possibly attributed to the discrepancies in sludge properties, ozonation parameters and test methods. However, it was clearly observed that polysaccharides contents increased continuously in sludge S-EPS and LB-EPS, indicating the solubilization and release of polysaccharides of TB-EPS and sludge cells, in consistence with the reported literature (Zhang et al., 2009; Zhang et al., 2016). Especially, at the ozone dosage of 60 mg/gTS, sludge S-EPS, LB-EPS and TBEPS polysaccharides changed from 65.41 mg/L, 84.15 mg/L, 156.79 mg/L to 1584.36 mg/L, 259.94 mg/L and 121.80 mg/L, respectively. Hence, during ozonation, the polysaccharide degradation rate was much lower than the polysaccharide solubilization rate in sludge S-EPS and LB-EPS. Through CT conditioning, EPS protein was further reduced, as shown in Fig. 6(c) and (d). Simultaneously, EPS polysaccharide content decreased to some extent, especially in S-EPS and LB-EPS. Thus, CT could efficaciously neutralize, flocculate and precipitate partially released ozonation biopolymers, increasing sludge particle size to a certain extent. Overall, the main roles of sludge ozonation and CT conditioning on sludge properties are shown in Fig. 7.
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Table 1 Heavy metals (As, Cd, Cr, Cu, Ni, Pb and Zn) contents in dewatered sludge cake of raw sludge and the treated sludge with 60 mg/gTS ozone treatment and 20 mg/gTS CT conditioning. Sample
As
Raw sludge cake Treated sludge cake Discharge standard (GB18918-2002) Agricultural A level (GB4284-2018)
25.77 1.58 22.43 130.24 24.93 19.18 650.42 14.20 1.48 12.94 184.48 16.49 5.87 516.81 75 5 600 800 100 300 2000 30
Cd
3
Cr
500
Cu
500
Ni
100
Pb
300
Zn
1200
Unit: mg/kg.
3.2.5. Variations of the heavy metal contents and chemical speciation in sludge cake In order to further assess the subsequent disposal of dewatered sludge, the heavy metal contents in dewatered sludge cake treated by two-step approach were analyzed in comparison with these of raw sludge. As shown in Table 1, after ozonation and CT conditioning, the contents of heavy metals (As, Cd, Cr, Ni, Pb and Zn) in dewatered sludge cake decreased, revealing that via ozone treating, the insoluble heavy metals in raw sludge transformed to soluble ionic state and/or complex states, entering the filtrate. However, the increased content of Cu was possibly due to the chelating effect of Chitosan and/or the adsorption effect of EPS by complexation and ion exchange (Fang et al., 2010; Li and Bai, 2005). In comparison with discharge standard (GB18918-2002) and agricultural A level (GB4284-2018), heavy metals contents satisfied the discharge and agricultural requirements, thus the dewatered sludge could be considered agricultural land. Heavy metals commonly restrict the ultimate utilization of sludge resources due to the strong bioaccumulation and low biodegradability in organisms (Altaş, 2009). Their harm to the environment depends on not only the total content but also the chemical speciation (X. Yuan et al., 2011). Generally, heavy metals can be divided into four chemical speciation including acid soluble/exchangeable state, reducible state, oxidizable state and residual state (X. Yuan et al., 2011). The acid soluble/exchangeable state has extremely strong mobility; the reductive state possesses highly potential mobility based on environment conditions; the oxidizable state can be complexed by organic compounds and it has strong stability; the residue state existing in mineralized or stabilized sludge is hard to mobilize or be utilized biologically. Especially, the amount of acid soluble/exchangeable state fraction plus
Fig. 7. The main roles of sludge ozonation and CT conditioning on sludge properties.
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Fig. 8. Variations of chemical speciation of heavy metals (As, Cd, Cr, Cu, Ni, Pb and Zn) in dewatered sludge cake of (a) raw sludge and (b) the treated sludge with 60 mg/gTS ozone treatment and 20 mg/gTS CT conditioning.
reductive state fraction can characterize the sludge toxicity and bioavailability (Zhang et al., 2016). As shown in Fig. 8, the residue state proportions of heavy metals (As, Cd, Cr, Cu and Zn) reduced apparently in dewatered sludge cake with ozonation and CT conditioning, indicating that the chemical speciation of most heavy metals shifted, in consistence with the report showing that the decrease of the residual states of Cu and Zn after ozonation (Zhang et al., 2016). The increased proportions of acid soluble/exchangeable state and reductive state of As, Cd, Cr, Cu and Zn revealed that these heavy metals in conditioned sludge cake had higher toxicity and bioavailability, which was also observed in other reports (Zhang et al., 2017, 2016). In general, heavy metals have a preference for the organic substances by complexing for the high stability, such as Cu, Cr, Cd and Zn (Lasheen and Ammar, 2009; Wong et al., 2007). However, during ozonation, large amounts of organic matters were released and oxidized in sludge flocs, making heavy metals move and convert. In addition, since the acid soluble/exchangeable state of heavy metals was easily extracted under the acid condition, coincidently, acid was generated and sludge pH decreased after ozonation (Zhang et al., 2016). Hence, environmental risk of these heavy metals aggravated, and required pretreatments were needed before further applications, such as passivation and stabilization (Wang et al., 2013). Moreover, the chemical speciation of Ni shifted little after conditioning, and the acid soluble/exchangeable state accounted for 26–32%, implying a certain degree of mobility, which was similar to the previous report (Zhang et al., 2017). For raw sludge cake or conditioned sludge cake, Pb mainly distributed in the residual state accounting for 73–77%, followed by the reduced state with the proportion of 15–20%. This result was in consistence with the reported literature (Chen et al., 2008; Zhang et al., 2017). The heavy metal of residual state was commonly embedded in aluminosilicate minerals, which were tough to break down (Nemati et al., 2011), and hence, Pb was relatively stabilized. 4. Conclusions For achieving environmentally friendly sludge deep-dewatering, a new two-step approach of ozonation and CT conditioning was examined to be feasible. Sludge CST and Wc of the dewatered sludge cake could obviously decrease from 196.3 s and 84.7% of raw sludge to 15.8 s and 57.5%, respectively, under the 60 mg/gTS ozone and 20 mg/gTS CT conditioning. Sludge viscosity, zeta potential and particle size decreased, and sludge EPS increased dramatically in the ozonation step. Subsequently, CT efficaciously flocculated released bio-polymeric colloids via charge neutralization and increased sludge particle size, which compressed sludge flocs and promoted the spread of interstitial
water, but sludge viscosity increased slightly. In addition, the contents of heavy metals (As, Cd, Cr, Ni, Pb and Zn) in conditioned dewatered sludge cake were detected to decrease and satisfy the A level of agricultural land (GB4284-2018). However, the proportions of acid soluble/exchangeable state and reductive state of heavy metals shifted pronouncedly, indicating the higher toxicity and bioavailability, except Pb. Accordingly, the pretreatments, such as passivation and stabilization, were required for the heavy metals in conditioned sludge cake before further applications. Although this paper offered a novel two-step approach to win the sludge deep-dewatering without any secondary pollution, further investigations on the scale experiment and the resource utilization of conditioned sludge cake should be conducted to thoroughly evaluate the practical value. Acknowledgements This work was supported by the Major Science and Technology Program for Water Pollution Control and Treatment of China (No. 2017ZX07403002) and National Natural Science Foundation of China (No. 21876110). References Altaş, L., 2009. Inhibitory effect of heavy metals on methane-producing anaerobic granular sludge. J. Hazard. Mater. 162, 1551–1556. APHA, 1998. Standard Methods for the Examination of Water and Wastewater. 20th ed. American Public Health Association, Washington, DC, USA. Bradford, M.M., 1976. A rapid and sensitive method for the quantitation of microgram quantities of protein utilizing the principle of protein-dye binding. Anal. Biochem. 72, 248–254. Bratskaya, S., Schwarz, S., Chervonetsky, D., 2004. Comparative study of humic acids flocculation with chitosan hydrochloride and chitosan glutamate. Water Res. 38, 2955–2961. Chen, Y., Yang, H., Gu, G., 2001. Effect of acid and surfactant treatment on activated sludge dewatering and settling. Water Res. 35, 2615–2620. Chen, M., Li, X.-m., Yang, Q., Zeng, G.-m., Zhang, Y., Liao, D.-X., et al., 2008. Total concentrations and speciation of heavy metals in municipal sludge from Changsha, Zhuzhou and Xiangtan in middle-south region of China. J. Hazard. Mater. 160, 324–329. Cooper, D., Pillon, D., Mulligan, C., Sheppard, J., 1986. Biological additives for improved mechanical dewatering of fuel-grade peat. Fuel 65, 255–259. Deng, W., Ma, J., Xiao, J., Wang, L., Su, Y., 2019. Orthogonal experimental study on hydrothermal treatment of municipal sewage sludge for mechanical dewatering followed by thermal drying. J. Clean. Prod. 209, 236–249. Fang, L., Huang, Q., Wei, X., Liang, W., Rong, X., Chen, W., et al., 2010. Microcalorimetric and potentiometric titration studies on the adsorption of copper by extracellular polymeric substances (EPS), minerals and their composites. Bioresour. Technol. 101, 5774–5779. Feng, X., Deng, J., Lei, H., Bai, T., Fan, Q., Li, Z., 2009. Dewaterability of waste activated sludge with ultrasound conditioning. Bioresour. Technol. 99, 1074–1081. Frølund, B., Palmgren, R., Keiding, K., Nielsen, P.H., 1996. Extraction of extracellular polymers from activated sludge using a cation exchange resin. Water Res. 30, 1749–1758.
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Wang, L.-F., Wang, L.-L., Li, W.-W., He, D.-Q., Jiang, H., Ye, X.-D., et al., 2014. Surfactantmediated settleability and dewaterability of activated sludge. Chem. Eng. Sci. 116, 228–234. Wang, D., Zhao, T., Yan, L., Mi, Z., Gu, Q., Zhang, Y., 2016. Synthesis, characterization and evaluation of dewatering properties of chitosan-grafting DMDAAC flocculants. Int. J. Biol. Macromol. 92, 761–768. Wang, J., Chon, K., Ren, X., Wu, H., Kou, Y., Hwang, M.-H., et al., 2019. Combined use of polymeric ferric sulfate and chitosan as a conditioning aid for enhanced digested sludge dewatering. Environ. Technol. 40, 2695–2704. Wojciechowska, E., 2005. Application of microwaves for sewage sludge conditioning. Water Res. 39, 4749–4754. Wong, J., Li, K., Zhou, L., Selvam, A., 2007. The sorption of Cd and Zn by different soils in the presence of dissolved organic matter from sludge. Geoderma 137, 310–317. Wu, B., Ni, B.J., Horvat, K., Song, L., Chai, X., Dai, X., et al., 2017. Occurrence state and molecular structure analysis of extracellular proteins with implications to the dewaterability of waste activated sludge. Environ. Sci. Technol. 51. Yuan, H., Zhu, N., Song, F., 2011a. Dewaterability characteristics of sludge conditioned with surfactants pretreatment by electrolysis. Bioresour. Technol. 102, 2308–2315. Yuan, X., Huang, H., Zeng, G., Li, H., Wang, J., Zhou, C., et al., 2011b. Total concentrations and chemical speciation of heavy metals in liquefaction residues of sewage sludge. Bioresour. Technol. 102, 4104–4110. Zemmouri, H., Mameri, N., Lounici, H., 2015. Chitosan use in chemical conditioning for dewatering municipal-activated sludge. Water Sci. Technol. 71, 810–816. Zhang, G., Yang, J., Liu, H., Zhang, J., 2009. Sludge ozonation: disintegration, supernatant changes and mechanisms. Bioresour. Technol. 100, 1505–1509. Zhang, J., Xia, W., Liu, P., Cheng, Q., Tahi, T., Gu, W., et al., 2010. Chitosan modification and pharmaceutical/biomedical applications. Mar. Drugs 8, 1962–1987. Zhang, J., Zhang, J., Tian, Y., Li, N., Kong, L., Sun, L., et al., 2016. Changes of physicochemical properties of sewage sludge during ozonation treatment: correlation to sludge dewaterability. Chem. Eng. J. 301, 238–248. Zhang, J., Tian, Y., Zhang, J., Li, N., Kong, L., Yu, M., et al., 2017. Distribution and risk assessment of heavy metals in sewage sludge after ozonation. Environ. Sci. Pollut. Res. 24, 5118–5125. Zhao, Y., Yin, J., Yu, H., Han, N., Tian, F., 2007. Observations on ozone treatment of excess sludge. Water Sci. Technol. 56, 167–175. Zhen, G., Lu, X., Zhao, Y., Chai, X., Niu, D., 2012. Enhanced dewaterability of sewage sludge in the presence of Fe(II)-activated persulfate oxidation. Bioresour. Technol. 116, 259–265. Zhen, G.Y., Lu, X.Q., Li, Y.Y., Zhao, Y.C., 2013. Innovative combination of electrolysis and Fe (II)-activated persulfate oxidation for improving the dewaterability of waste activated sludge. Bioresour. Technol. 136, 654–663. Zhen, G., Lu, X., Su, L., Kobayashi, T., Kumar, G., Zhou, T., et al., 2018. Unraveling the catalyzing behaviors of different iron species (Fe2+ vs. Fe0) in activating persulfatebased oxidation process with implications to waste activated sludge dewaterability. Water Res. 134, 101–114. Zhen, G., Wang, J., Lu, X., Su, L., Zhu, X., Zhou, T., et al., 2019. Effective gel-like floc matrix destruction and water seepage for enhancing waste activated sludge dewaterability under hybrid microwave-initiated Fe (II)-persulfate oxidation process. Chemosphere 221, 141–153. Zhou, X., Jiang, G., Zhang, T., Wang, Q., Xie, G.-j., Yuan, Z., 2015. Role of extracellular polymeric substances in improvement of sludge dewaterability through peroxidation. Bioresour. Technol. 192, 817–820.
Contents lists available at ScienceDirect
Science of the Total Environment journal homepage: www.elsevier.com/locate/scitotenv
Insight into a new two-step approach of ozonation and chitosan conditioning for sludge deep-dewatering Dongdong Ge a, Wenrui Zhang a, Chang Bian a, Haiping Yuan a, Nanwen Zhu a,b,⁎ a b
School of Environmental Science & Engineering, Shanghai Jiao Tong University, Shanghai 200240, China Shanghai Institute of Pollution Control and Ecological Security, Shanghai 200092, China
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
• A new two-step approach was proposed for sludge deep-dewatering. • CT neutralized and flocculated negative ozonated sludge colloids efficiently. • Heavy metals contents of conditioned sludge cake met A level of agricultural land. • Most heavy metals' chemical speciation altered to higher toxicity and bioavailability.
a r t i c l e
i n f o
Article history: Received 12 June 2019 Received in revised form 1 August 2019 Accepted 20 August 2019 Available online 22 August 2019 Editor: Deyi Hou Keywords: Sludge deep-dewatering Ozonation Chitosan (CT) Sludge properties Heavy metals
a b s t r a c t Sludge deep-dewatering, capable of reducing water content (Wc) of sludge cake under 60%, is one of the current hot topics in sludge treatment. In this study, a new environmentally friendly two-step approach of ozonation and chitosan (CT) conditioning was proposed and examined to be practicable for sludge deep-dewatering. With 60 mg/gTS ozone and 20 mg/gTS CT conditioning, sludge capillary suction time (CST) and Wc of the dewatered sludge cake decreased from 196.3 s and 84.7% of raw sludge to 15.8 s and 57.5%, respectively. Ozonation treatment could efficiently crack sludge flocs and cells, and release biopolymers, causing the decreases in viscosity, zeta potential and particle size. Subsequently, CT ameliorated the sludge dewaterability successfully by neutralizing negative charges and flocculating colloids to promote the spread of interstitial water. Furthermore, the contents of heavy metals (As, Cd, Cr, Ni, Pb and Zn) in conditioned sludge cake decreased obviously except Cu, but all detected heavy metals contents satisfied the A level of agricultural land (GB4284-2018). For chemical speciation of heavy metals, the proportions of the acid soluble/exchangeable state and the reductive state increased apparently, implying higher toxicity and bioavailability, except Pb. Hence, pretreatments were required to reduce the environmental risk of heavy metals in conditioned sludge cake prior to a further utilization. © 2019 Elsevier B.V. All rights reserved.
1. Introduction
⁎ Corresponding author at: School of Environmental Science & Engineering, Shanghai Jiao Tong University, Shanghai 200240, China. E-mail address: [email protected] (N. Zhu).
https://doi.org/10.1016/j.scitotenv.2019.134032 0048-9697/© 2019 Elsevier B.V. All rights reserved.
In China, the annual municipal waste activated sludge (WAS) production has exceeded 42 million tons and is still growing (Ge et al., 2019a). To reduce the sludge output of sewage treatment plant, researchers are beginning to looking for a deep-dewatering technique
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capable of reducing water content (Wc) of sludge cake under 60%. However, so far, due to the high WAS moisture of 95–99% and unsatisfactory dewaterability, the Wc of dewatered sludge cake after mechanical dewatering with conventional coagulant and/or flocculant still remains high at around 75–80% (Zhen et al., 2018). Hence, sludge conditioning becomes particularly important for further enhancing WAS dewaterability. A wide variety of conditioning techniques have been investigated for WAS dewatering such as thermal hydrolysis (Deng et al., 2019), microwave (Wojciechowska, 2005; Zhen et al., 2019), electrolysis (H. Yuan et al., 2011), ultrasound (Feng et al., 2009), bioleaching (Huo et al., 2014), and advanced oxidation processes (AOPs) (Liu et al., 2016; Lu et al., 2003; Zhen et al., 2018). Nevertheless, due to the highly hydrophilic colloid structure of sludge flocs and rigid sludge cell walls making sludge dehydration extremely hard, comprehensive disintegrations of sludge extracellular polymeric substances (EPS) and breakdowns of sludge cell walls are greatly imperative for the thorough release of intracellular and chemically bound water. Noted worthily, lots of the single technique such as electrolysis, ultrasonication, and bioleaching remain in decomposing sludge EPS to enhance sludge dewatering due to their mild degradation capacity and overtreatment will cause the dewaterability deterioration again. Differently, AOPs represented by Fenton and Fe2+/persulfate processes can quickly crack sludge flocs and cell walls to release intracellular substances and highly degrade sludge EPS to harvest efficient sludge deepdewatering via the strongly oxidative free radicals and re-flocculation effect (Zhen et al., 2018). However, Fenton reagents exerted an excellent role at the demanding operating condition of pH 3.0 (Lu et al., 2003); Fe2+/persulfate reagents of 1.5 mmol/gVSS Fe2+ and 1.2 mmol/gVSS persulfate could achieve the desired sludge dewaterability rapidly with 1 min (Zhen et al., 2012), but cost more. Moreover, the additive Fe2+ easily induces filter cloth clogging, pump wear, equipment life-shortening and sludge calorific value reduction in actual operation. Therefore, it is greatly necessary to seek a more feasible, environmentally friendly and economical approach for sludge deep-dewatering. Ozonation, as one promising technique for efficiently cracking sludge cells with no secondary toxic by-products and moderate operating cost, has been revealed to cause 60% sludge suspended solid reduction and the leakages of carbon, nitrogen and phosphorus from sludge
substrate into supernatant (Sang-Tian et al., 2009). Unfortunately, the physicochemical properties of ozonation sludge altered dramatically and sludge dewaterability deteriorated seriously (Zhang et al., 2016; Zhao et al., 2007). Thus, it is quite meaningful to consider adding a subsequent treating following sludge ozonation to reverse the major factors affecting sludge dewaterability for realizing a nontoxic and harmless dewatering process. Considering the solubilization of sludge biopolymers into small molecular organic matters with negative charges and the high cellular lysis efficiency causing the sludge particle size reduction after ozonation, we deduce that charge neutralization and flocculation are possibly favorable for further dewaterability amelioration of ozonation sludge. Chitosan (CT), as the only natural and renewable cationic biopolymer with the outstanding properties of non-toxicity, biocompatibility as well as biodegradability, has been extensively adopted in food processing, pharmaceutical application and sewage treatment (No and Meyers, 2000; Rinaudo, 2006; Zhang et al., 2010). It is an aminopolysaccharide with a linear and semi-rigid structure and efficaciously functions as the adsorbent and/or flocculant for capturing metal ions and polymers such as protein by active hydroxyl and amino groups (No and Meyers, 2000; Zemmouri et al., 2015), so CT has great potential to be the alternative replacement of synthetic organic flocculant (PAM), and the formed CT sludge can serve as additive in fertilizers and animal feed (Bratskaya et al., 2004). At present, there are a few reports with respect to the application of CT in sludge dewatering. However, the investigated contents only focused on the dewaterability effects (Cooper et al., 1986; Zemmouri et al., 2015), and lacked the research concerning sludge properties which were greatly instrumental in exploring dewatering mechanism by CT. Other researchers devoted to the synthesis of chitosan-based flocculant and emphasized the characteristics of the modified materials without the discussion regarding their roles on sludge properties (Huang et al., 2016; Wang et al., 2016). Hence, attention must be paid on not only the apparent dewaterability but also the underlying conditioning mechanism. In this study, a novel attempt of two-step approach associating sludge ozonation with CT conditioning was conducted to realize sludge deep-dewatering in an environmentally friendly manner, of which flow chart was shown in Fig. 1. The sludge dewaterability was characterized
Fig. 1. Flow chart of the new two-step approach of ozonation and CT conditioning for sludge deep-dewatering.
D. Ge et al. / Science of the Total Environment 697 (2019) 134032
by capillary suction time (CST) and Wc of dewatered sludge cake which was acquired by a laboratory filter press for being closer to the actual operating value. Sludge properties including viscosity, zeta potential, particle size, EPS (protein and polysaccharide) contents as well as the contents and chemical speciation of heavy metals (As, Cd, Cr, Cu, Ni, Pb and Zn) in dewatered sludge cake were especially monitored for a comprehensive insight into the mechanism of sludge deep-dewatering by this two-step method.
2. Materials and methods
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2.2. Experimental process Sludge sample of 200 mL fed into the reactor was subjected to ozonation and the ozone dosage was calculated as the quantity of inlet ozone quantity minus outflow ozone quantity. Various ozone dosages (20 mg/gTS, 40 mg/gTS, 60 mg/gTS, 80 mg/gTS and 100 mg/gTS) were added by adjusting reaction time. Then, the obtained ozonation sludge samples continually stirred overnight for completely consuming the residual ozone. 150 mL ozonation sludge was conditioned using chitosan solution on a magnetic stirring apparatus with the stirring speed of 350 rpm for 3 min and the sludge samples without ozonation treating and/or CT conditioning were set as control groups.
2.1. Materials 2.3. Analysis methods WAS was gained from the secondary settling tank of a municipal sewage treatment plant, which adopted an anaerobic-anoxic-oxic process in Shanghai, China. The collected sludge was then screened through 1.0 mm sieve to discard the gravel and hairs and subsequently centrifuged for thickening the WAS with about 3% total solid (TS). The characteristics of the prepared sludge were as follow: total solid (TS) of 31.2 ± 0.5 g/L, volatile solid (VS) of 18.9 ± 0.6 g/L, pH value of 7.24 ± 0.02, CST of 196.3 ± 3.1 s and Wc of dewatered sludge cake of (84.7 ± 0.1)% prior to dewatering experiments. Ozone was produced using an ozone generator with pure oxygen as gas source and bubbled into the ozonation reactor made of plexiglass through the micropore aeration plate at the bottom of the reactor. The inlet and outflow ozone concentrations were monitored by a digital ozone concentration detector, and the inlet ozone concentration was set as 35 mg/L and the inlet flow was 1 L/min. CT (deacetylation degree ≥95%, viscosity of 100–200 mPa·s) was purchased from Macklin Co. Ltd. The 1.5% (wt%) chitosan solution prepared with 5% acetic acid solution was employed to condition sludge.
2.3.1. Sludge dewaterability The CST value was detected using a 304B CST instrument (model, Triton, UK). The water content (Wc) of the dewatered sludge cake gained through a laboratory filter press with 100 mL sludge sample at a pressure of 0.6 MPa for 10 min was calculated according to the standard method (APHA, 1998). 2.3.2. EPS extraction and analysis Sludge EPS extraction was abided by the method described in Zhen's research (Zhen et al., 2013). Various extracted supernatant samples defined as slime EPS (S-EPS), loosely bound EPS (LB-EPS) and tightly bound EPS (TB-EPS) were further filtered through 0.45 μm microfiber membrane for protein and polysaccharide determinations. 2.3.3. Sludge properties TS and VS of sludge samples were determined according to the standard methods (APHA, 1998). Sludge viscosity was monitored by a rotary
Fig. 2. Variations of (a) sludge CST at various ozone dosages; variations of (b) sludge CST and (c) Wc of dewatered sludge cake with various ozone dosages and 20 mg/gTS conditioning; variations of (d) sludge CST and (e) Wc of dewatered sludge cake with 60 mg/gTS ozone treatment and CT conditioning at various CT dosages.
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Fig. 3. Variations of sludge viscosity with (a) various ozone dosages and (b) 60 mg/gTS ozone treatment and CT conditioning at various CT dosages.
viscosimeter (NDJ-8S). Sludge particle size distribution was analyzed by a Malvern UK Mastersizer 3000. The zeta potential was determined with a Beckman Coulter Delsa™ Nano instrument. The pH value was determined with a digital pH-meter (pHs-3C). EPS protein was determined by the Bradford method (Bradford, 1976), using bovine serum albumin (BSA) as the standard. EPS polysaccharide was determined by the anthrone-sulfuric acid method (Hassid and Abraham, 1957), using glucose as the standard. The concentrations of heavy metals (As, Cd, Cr, Cu, Ni, Pb and Zn) were determined using an Agilent 5110 ICP-OES, and the chemical speciation of heavy metals was analyzed using BCR sequential extraction method (X. Yuan et al., 2011). 3. Results and discussion 3.1. The role of ozonation and CT conditioning on sludge dewaterability CST is valuably indicative of WAS dewaterability, especially sludge filterability (Chen et al., 2001). As shown in Fig. 2(a), the raw sludge CST was 196.3 s. With the treatment of ozone ranging from 20 mg/gTS to 80 mg/gTS, sludge CST deteriorated from 259.5 s to 712.3 s pronouncedly, which implied that sludge filterability turned poor and the moisture trapped in sludge became harder to remove. In order to further enhance the dewaterability of ozonated sludge, 20 mg/gTS CT was fed in the sludge at various ozone dosages. As shown in Fig. 2(b), after step 2 of CT conditioning, sludge CST further ameliorated apparently, revealing that CT successfully altered partial sludge properties
impeding sludge filterability after ozonation. Therefore, CT was also suitable for the conditioning of ozonated sludge, not only traditional municipal and digested sludge reported previously (Wang et al., 2019; Zemmouri et al., 2015). However, the more ozone dosage added in step 1, the larger CST of CT-conditioned sludge became, suggesting that the sludge with a higher ozonated degree was more difficult to adjust. In practical operation, Wc of dewatered sludge cake, as the most intuitive and meaningful indicator, reflected the degree of sludge volume reduction (Ge et al., 2019b). As shown in Fig. 2(c), with the conditioning of various ozone dosage and 20 mg/gTS CT, Wc of dewatered sludge cake firstly decreased and then slightly increased, implying that CT effectually strengthened the separation of solid sludge and water liquid after sludge ozonation. Noted worthily, when the ozone dosage was 60 mg/gTS, the Wc of dewatered sludge cake could be reduced to 57.5% b60%, indicating that this two-step approach of ozonation and CT conditioning was a promising and powerful technique to accomplish sludge deep-dewatering. Furthermore, the effect of various CT dosages on sludge dewaterability was further investigated under 60 mg/g TS ozone. As shown in Fig. 2(d) and (e), sludge CST and Wc of dewatered sludge cake could be reduced to b50 s and 60% with the CT dosage N15 mg/gTS, while the further cuts on CST and Wc would be slight with the CT dosage beyond 20 mg/gTS. Here, the CT dosage introduced in ozonated sludge was slightly higher than the report CT dosage used in municipal and digested sludge as well as peat (Cooper et al., 1986; Wang et al., 2019; Zemmouri et al., 2015), primarily resulting from the special ozonation sludge properties, which would be discussed in later section.
Fig. 4. Variations of sludge zeta potential with (a) various ozone dosages and (b) 60 mg/gTS ozone treatment and CT conditioning at various CT dosages.
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3.2. The role of ozonation and CT conditioning on sludge properties 3.2.1. Variations of sludge viscosity with ozonation and CT conditioning Sludge viscosity, as a valuable factor influencing WAS dewatering, has been observed to be positively correlated with sludge dewaterability in the previous reports (Li and Yang, 2007; Zhen et al., 2012). Interestingly, as shown in Fig. 3(a), with the increased ozone dosage ranging from 0 to 80 mg/gTS in step 1, sludge viscosity declined constantly from 412.0 mPa·s to 57.5 mPa·s, but sludge dewaterability worsened distinctly as depicted in Fig. 2(a). Further, as shown in Fig. 3 (b), with the added CT dosage ranging from 15.0 mg/gTS to 22.5 mg/gTS in step 2, sludge viscosity gradually decreased and then rose slightly, implying that the helpful CT was within a certain dosage range and excessive CT (N20 mg/gTS) led to the increased sludge viscosity again. Noted worthily, sludge viscosity increased by at least 6.0 mPa·s corresponding to the CT dosage of 20 mg/gTS, whereas the ozonated sludge dewaterability improved from CST of 692.4 s to CST
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of 15.8 s significantly. The occurred negative correlations between sludge viscosity and sludge dewaterability in the two steps were not consistent with the mentioned report (Zhen et al., 2012). The reason was probably that sludge was subjected to ozone oxidation and organic conditioner, different from the Fe2+/persulfate process plus inorganic re-flocculation in Zhen's investigation (Zhen et al., 2018). Fe2+/persulfate could effectively break down the sludge cells and EPS by generated strongly powerful sulfate radicals and hydroxyl radicals, and the flocculation effect of iron ions further largely remove sludge EPS (Zhen et al., 2012, 2018). Thus, sludge viscosity reduced and WAS dewaterability enhanced dramatically in Fe2+/persulfate process (Zhen et al., 2012). However, during the ozone oxidation, large sludge granules were gradually oxidized into small particles, inducing the continual release of bound water and sludge biopolymers as well as the increase of sludge negative charge, which were concretely discussed in the later sections. Hence, sludge viscosity reduced to some extent, but more released and stabilized biopolymers afforded more binding sites for water in
Fig. 5. Variations of sludge particle size with (a) various ozone dosages and (b) 60 mg/gTS ozone treatment and CT conditioning at various CT dosages.
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the sludge matrix, and sludge dewaterability worsened. Moreover, CT, as an organic conditioner, could flocculate sludge biocolloid via adsorption, crosslinking and bridging (Huang et al., 2016), which was different from the function of neutralizing charges and compressing double electric layer of inorganic coagulant (Niu et al., 2013). Similarly, tannic acid, as another natural organic conditioner, was also observed to enhance sludge dewaterability but increase sludge viscosity to a certain extent (Ge et al., 2019b). 3.2.2. Variations of sludge zeta potential with ozonation and CT conditioning Zeta potential can reflect the electrically charged sludge colloid accurately and the elimination of negative or positive charges would be favorable for the enhancement of WAS dewaterability (Ge et al., 2019b; Jin et al., 2004). As shown in Fig. 4(a), the zeta potential of raw sludge was −16.81 mV. With the increased ozone dosage, the sludge zeta potential decreased obviously. As ozone dosage was 60 mg/gTS, sludge zeta potential dropped to −24.42 mV and then altered mildly. This was because the negatively charged organic matters were continually released from the internal sludge cells and flocs into the external space of sludge during ozonation, and the released biopolymers carrying identical electrical charges repelled with each other, which further blocked the flow of water and caused poor dewaterability (Liu et al., 2001; Zhang et al., 2009). But to a certain level, the oxidized rate balanced the released rate, sludge zeta potential changed slightly. Further, as shown in Fig. 4(b), with the increased CT, sludge zeta potential gradually tended to 0. Especially, when the CT dosage was 20 mg/gTS, sludge zeta potential reached to −1.59 mV which was close to neutrality. The
results implied that CT, as a cationic electrolyte, could remarkably neutralize negative charges of organic biopolymers, such as protein and polysaccharide, and the sludge biocolloid would gradually destabilize, agglomerate and settle down (Wang et al., 2016).
3.2.3. Variations of sludge particle size with ozonation and CT conditioning Sludge particle size is closely related to sludge dewaterability (Karr and Keinath, 1978; Shao et al., 2009). As shown in Fig. 5(a), sludge particle size decreased significantly via ozonation, and especially, with 60 mg/gTS ozone treatment, sludge D50 and D90 decreased from 54.9 μm and 138 μm of raw sludge to 46.2 μm and 120 μm, respectively, which revealed that ozone efficiently cracked and oxidized large sludge flocs into small particles (Zhao et al., 2007). Subsequently, through the CT conditioning, sludge particle enlarged pronouncedly with the increased CT dosage, as depicted in Fig. 5(b). It was distinctly observed that, as the CT dosage was 20 mg/gTS, sludge D50 and D90 increased by 20.3 μm and 135 μm, respectively. However, in step 1, ozonation disruption would increase sludge surface area and expose more hydrophilic groups, strengthening the affinity of sludge colloids to water and further deteriorating sludge dewatering (Zhang et al., 2016; Zhao et al., 2007). In step 2, CT conditioning efficaciously flocculated the released colloids and enlarged ozonated sludge particle size, promoting the compression of aggregates and the spread of trapped interstitial water, which was consistent with the report (Cooper et al., 1986). Interestingly, it was clearly observed that a large number of aggregated flocs were formed, and water gradually spilled out from the agglomerations during CT conditioning experiments.
Fig. 6. Variations of (a) EPS protein and (b) EPS polysaccharide with various ozone dosages; variations of (c) EPS protein and (d) EPS polysaccharide with 60 mg/gTS ozone treatment and CT conditioning at various CT dosages.
D. Ge et al. / Science of the Total Environment 697 (2019) 134032
3.2.4. Variations of EPS contents with ozonation and CT conditioning Sludge EPS is secreted by the bacteria embedded in sludge flocs and protected their living surroundings from the exterior environment (Zhou et al., 2015). It mainly consists of protein, polysaccharide, lipid and nucleic acid, etc. (Frølund et al., 1996). Among the biopolymers, protein and polysaccharide account for 70–80% of the extracellular organic carbon (Neyens et al., 2004), and especially, EPS protein has been reported as the major factor governing WAS dewaterability (Shao et al., 2009; Wang et al., 2014; Wu et al., 2017). As depicted in Fig. 6(a) and (b), the biopolymers of sludge could be pronouncedly released into EPS by ozonation destroying sludge flocs and cell walls. At low ozone dosage (40 mg/gTS), S-EPS protein increased from 4.97 mg/L (raw sludge) to 36.03 mg/L as a result of the migration of protein from sludge inside to outside and sludge dewaterability worsened dramatically. With the ozone dosage over 40 mg/gTS, all of the protein contents in S-EPS, LB-EPS and TB-EPS decreased gradually, suggesting that protein degradation rate was higher than protein solubilization rate in the three EPS stratifications. Especially, with the ozone dosage of 60 mg/gTS, the proteins in sludge S-EPS, LB-EPS and TB-EPS altered from 4.97 mg/L, 23.11 mg/L and 28.61 mg/L of raw sludge to 10.20 mg/L, 1.49 mg/L and 2.27 mg/L, respectively, revealing that ozone could efficaciously dissolve, transfer, degrade and even mineralize sludge proteins. The result was different from the previous report showing the continual increase of EPS protein in ozonation treatment (Zhang et al., 2016), which was possibly attributed to the discrepancies in sludge properties, ozonation parameters and test methods. However, it was clearly observed that polysaccharides contents increased continuously in sludge S-EPS and LB-EPS, indicating the solubilization and release of polysaccharides of TB-EPS and sludge cells, in consistence with the reported literature (Zhang et al., 2009; Zhang et al., 2016). Especially, at the ozone dosage of 60 mg/gTS, sludge S-EPS, LB-EPS and TBEPS polysaccharides changed from 65.41 mg/L, 84.15 mg/L, 156.79 mg/L to 1584.36 mg/L, 259.94 mg/L and 121.80 mg/L, respectively. Hence, during ozonation, the polysaccharide degradation rate was much lower than the polysaccharide solubilization rate in sludge S-EPS and LB-EPS. Through CT conditioning, EPS protein was further reduced, as shown in Fig. 6(c) and (d). Simultaneously, EPS polysaccharide content decreased to some extent, especially in S-EPS and LB-EPS. Thus, CT could efficaciously neutralize, flocculate and precipitate partially released ozonation biopolymers, increasing sludge particle size to a certain extent. Overall, the main roles of sludge ozonation and CT conditioning on sludge properties are shown in Fig. 7.
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Table 1 Heavy metals (As, Cd, Cr, Cu, Ni, Pb and Zn) contents in dewatered sludge cake of raw sludge and the treated sludge with 60 mg/gTS ozone treatment and 20 mg/gTS CT conditioning. Sample
As
Raw sludge cake Treated sludge cake Discharge standard (GB18918-2002) Agricultural A level (GB4284-2018)
25.77 1.58 22.43 130.24 24.93 19.18 650.42 14.20 1.48 12.94 184.48 16.49 5.87 516.81 75 5 600 800 100 300 2000 30
Cd
3
Cr
500
Cu
500
Ni
100
Pb
300
Zn
1200
Unit: mg/kg.
3.2.5. Variations of the heavy metal contents and chemical speciation in sludge cake In order to further assess the subsequent disposal of dewatered sludge, the heavy metal contents in dewatered sludge cake treated by two-step approach were analyzed in comparison with these of raw sludge. As shown in Table 1, after ozonation and CT conditioning, the contents of heavy metals (As, Cd, Cr, Ni, Pb and Zn) in dewatered sludge cake decreased, revealing that via ozone treating, the insoluble heavy metals in raw sludge transformed to soluble ionic state and/or complex states, entering the filtrate. However, the increased content of Cu was possibly due to the chelating effect of Chitosan and/or the adsorption effect of EPS by complexation and ion exchange (Fang et al., 2010; Li and Bai, 2005). In comparison with discharge standard (GB18918-2002) and agricultural A level (GB4284-2018), heavy metals contents satisfied the discharge and agricultural requirements, thus the dewatered sludge could be considered agricultural land. Heavy metals commonly restrict the ultimate utilization of sludge resources due to the strong bioaccumulation and low biodegradability in organisms (Altaş, 2009). Their harm to the environment depends on not only the total content but also the chemical speciation (X. Yuan et al., 2011). Generally, heavy metals can be divided into four chemical speciation including acid soluble/exchangeable state, reducible state, oxidizable state and residual state (X. Yuan et al., 2011). The acid soluble/exchangeable state has extremely strong mobility; the reductive state possesses highly potential mobility based on environment conditions; the oxidizable state can be complexed by organic compounds and it has strong stability; the residue state existing in mineralized or stabilized sludge is hard to mobilize or be utilized biologically. Especially, the amount of acid soluble/exchangeable state fraction plus
Fig. 7. The main roles of sludge ozonation and CT conditioning on sludge properties.
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Fig. 8. Variations of chemical speciation of heavy metals (As, Cd, Cr, Cu, Ni, Pb and Zn) in dewatered sludge cake of (a) raw sludge and (b) the treated sludge with 60 mg/gTS ozone treatment and 20 mg/gTS CT conditioning.
reductive state fraction can characterize the sludge toxicity and bioavailability (Zhang et al., 2016). As shown in Fig. 8, the residue state proportions of heavy metals (As, Cd, Cr, Cu and Zn) reduced apparently in dewatered sludge cake with ozonation and CT conditioning, indicating that the chemical speciation of most heavy metals shifted, in consistence with the report showing that the decrease of the residual states of Cu and Zn after ozonation (Zhang et al., 2016). The increased proportions of acid soluble/exchangeable state and reductive state of As, Cd, Cr, Cu and Zn revealed that these heavy metals in conditioned sludge cake had higher toxicity and bioavailability, which was also observed in other reports (Zhang et al., 2017, 2016). In general, heavy metals have a preference for the organic substances by complexing for the high stability, such as Cu, Cr, Cd and Zn (Lasheen and Ammar, 2009; Wong et al., 2007). However, during ozonation, large amounts of organic matters were released and oxidized in sludge flocs, making heavy metals move and convert. In addition, since the acid soluble/exchangeable state of heavy metals was easily extracted under the acid condition, coincidently, acid was generated and sludge pH decreased after ozonation (Zhang et al., 2016). Hence, environmental risk of these heavy metals aggravated, and required pretreatments were needed before further applications, such as passivation and stabilization (Wang et al., 2013). Moreover, the chemical speciation of Ni shifted little after conditioning, and the acid soluble/exchangeable state accounted for 26–32%, implying a certain degree of mobility, which was similar to the previous report (Zhang et al., 2017). For raw sludge cake or conditioned sludge cake, Pb mainly distributed in the residual state accounting for 73–77%, followed by the reduced state with the proportion of 15–20%. This result was in consistence with the reported literature (Chen et al., 2008; Zhang et al., 2017). The heavy metal of residual state was commonly embedded in aluminosilicate minerals, which were tough to break down (Nemati et al., 2011), and hence, Pb was relatively stabilized. 4. Conclusions For achieving environmentally friendly sludge deep-dewatering, a new two-step approach of ozonation and CT conditioning was examined to be feasible. Sludge CST and Wc of the dewatered sludge cake could obviously decrease from 196.3 s and 84.7% of raw sludge to 15.8 s and 57.5%, respectively, under the 60 mg/gTS ozone and 20 mg/gTS CT conditioning. Sludge viscosity, zeta potential and particle size decreased, and sludge EPS increased dramatically in the ozonation step. Subsequently, CT efficaciously flocculated released bio-polymeric colloids via charge neutralization and increased sludge particle size, which compressed sludge flocs and promoted the spread of interstitial
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