Feasibility and Mechanism of Combined Conditioning with Coagulant

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Research Article Cite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Feasibility and Mechanism of Combined Conditioning with Coagulant and Flocculant To Enhance Sludge Dewatering Junyuan Guo,* Cheng Chen, Shilin Jiang, and Yuling Zhou

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College of Resources and Environment, Chengdu University of Information Technology, No. 24 Block 1, Xuefu Road, Chengdu, Sichuan 610225, China ABSTRACT: Coagulation or flocculation is often individually used or in hybrid conditioning with other preconditioning for sludge dewatering. However, combined conditioning with coagulants and flocculants for sludge dewatering is rarely reported. In this study, dewatering performances of sludge with individual use of poly(aluminum chloride) (PACl; as the coagulant) and a biopolymer named “BF-ADSW” (as the flocculant) were investigated, and the feasibility of applying the coagulation− flocculation process using PACl and BF-ADSW for sludge dewatering and the related mechanism were explored. BFADSW was harvested from anaerobically digested swine wastewater (ADSW), and its yield was 2.98 g/L, meaning that 2.98 g of BF-ADSW can be harvested from 1 L of ADSW. After conditioning by PACl with a dose of 10% dry solids (DS), the sludge moisture content and settled volume after 30 min were decreased from the raw values of 98.5% and 96.1% to 80.8% and 74.2%, respectively, which were further decreased to 62.3% and 49.2% after further conditioning by 15% DS of BF-ADSW, respectively, indicating that sludge dewaterability and settleability can be obviously improved by PACl coagulation followed by BF-ADSW flocculation. Compared to sludge conditioning by individual PACl and BF-ADSW, reduction of bound water in the coagulation−flocculation process was the most efficient (2.24 g/g of DS), and release of sludge extracellular polymeric substances was the largest (32.2 mg/g of volatile suspended solids), resulting in the best dewatering performance. The synergistic effect of coagulation and flocculation, including charge neutralization and bridge aggregation, was the main mechanism that efficiently enhanced sludge dewatering. These findings demonstrated that coagulation−flocculation is a feasible process to improve sludge dewatering. KEYWORDS: Sludge dewatering, Coagulation−flocculation process, Poly(aluminum chloride) (PACl), Biopolymer, Anaerobically digested swine wastewater (ADSW)

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characteristics. It can ameliorate the filterability during the sludge dewatering process, but there was also a limitation for deep sludge dewatering that the extracellular polymeric substances (EPSs) cannot be degraded effectively. Sludge EPSs, accounting for about 60−80% of the total sludge mass, have been considered as a key factor which significantly affects sludge dewatering.6,7 Some researchers reported that the bound EPSs (B-EPSs) were related to sludge dewatering,2 while others considered the soluble EPSs (S-EPSs) to be the influencing factor.8,9 Hence, it is essential to explore the feasibility of the combined use of coagulants and flocculants for sludge dewatering to achieve good filterability and efficiently reduce the water content, and it is necessary to clarify the possible mechanisms of improving sludge dewatering by detecting the EPS distribution and composition. In fact, the combined coagulation−flocculation is an efficient method in wastewater treatment facilities and is popular for removing

INTRODUCTION Waste-activated sludge from the secondary settling tank of wastewater treatment plants (WWTPs) generally contains over 99% water, which results in a high environmental burden, strong limitation of disposal routes.1 Thus, it is necessary to develop effective dewatering processes to reduce the sludge volume for disposal. Accordingly, sludge pretreatment prior to mechanical dewatering is required.2 Various conditioning pretreatments have been used to enhance sludge dewatering, in which coagulation or flocculation for wastewater sludge is often used alone or as hybrid conditioning with membrane biotechnologies for full-scale sludge dewatering.3,4 Ferric and aluminum salts were the commonly and widely used chemical coagulants prior to mechanical dewatering to improve the sludge dewatering efficiency in WWTPs. They can partly destroy sludge flocs and promote the release of bound water through charge neutralization.3 However, large amounts of inorganic ions were always carried into wastewaters by using these chemicals, which shorten the service life of wastewater treatment equipment.5 Biopolymer has been considered as a substitute due to its safety and environmentally friendly © XXXX American Chemical Society

Received: May 7, 2018 Revised: June 20, 2018

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DOI: 10.1021/acssuschemeng.8b02086 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering turbidity and organic matter.10−12 However, research on the combined chemical coagulation and biopolymer treatment on sludge dewatering is lacking, and the evolution of EPSs and morphological properties is still not clear. So far, for biopolymer production and application, with the aim of commercialization, efforts have been made to reduce the production cost.13−16 A large quantity of swine wastewater has been generated from the extensive swine breeding in China, which has become an increasingly severe pollution problem.17 Anaerobic digestion converts organic matter into biogas, allowing the production of renewable energy, which has been vigorously promoted by the Chinese government.18 After anaerobic treatment, there are still large amounts of organic matter and ammonia in anaerobically digested swine wastewater (ADSW) that cannot be discharged directly.19 In this case, ADSW could be used to cultivate microorganisms, and the metabolites could be a source to extract biopolymer, which is of academic and practical interests. Thus, the objectives of this study were (1) to harvest a biopolymer from cheap substrates, ADSW, (2) to investigate the effects of PACl and biopolymer on sludge dewatering performances separately, (3) to explore the feasibility of a combined coagulation−flocculation (PACl as the coagulant and biopolymer as the flocculant) process for sludge dewatering, (4) to deeply understand the EPS distribution and composition, and (5) to explore mechanisms for sludge dewatering among the coagulation−flocculation process. The biopolymer could be an environmentally friendly way to enhance sludge dewatering, the dynamic variation in distribution and composition of EPSs can clarify the dewatering mechanism, and the combined PACl coagulation and biopolymer flocculation may be a promising technology in sludge dewatering in WWTPs.

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chosen as the suspended solid, whose optical density (OD) was measured with a spectrophotometer (Unic-7230, Shanghai Lianhua Co., China) at 550 nm before and after treatment by the BF-ADSW. The control experiment was conducted in the same manner without adding BF-ADSW. The flocculating activity was calculated by the following equation:

FR =

B−A × 100 B

(1)

where FR is the flocculating activity and A and B are the OD values of the sample and control. Sludge Dewatering Test. PACl was selected as the coagulant, and BF-ADSW was selected as the flocculant to condition the biological sludge and further to enhance sludge dewatering. Three types of conditioning modes were conducted: individual use of PACl, individual use of BF-ADSW, and combination use of PACl and BFADSW (coagulation−flocculation process). The settled volume after 30 min (SV30, %), specific resistance to filtration (SRF), dry solids (DS) content, moisture content (MC), and capillary suction time (CST) of the sludge were chosen to evaluate the sludge dewatering performance. For each conditioning test, PACl/BF-ADSW with a range of 5−50% DS was first added into 100 mL of sludge, and then the mixture was stirred following the procedure of a rapid stir period for 1 min at 200 rpm followed by a slow-stir phase at 50 rpm for 9 min. After agitation, the conditioned sludge was transferred into a 100 mL graduated cylinder and settled for 30 min to measure SV30 in accordance with the standard method and to further evaluate the sludge settleability.22 DS and SRF were determined according to the equations cited in our previous study.23 MC of the dewatered sludge cake (formed by using Buchner funnel filtration at 0.04 MPa for 10 min) was identified in accordance with standard methods.22 The filterability of the sludge was estimated with a CST instrument (Fann440, Zhonghui Tiancheng Technology Co., Ltd., China). To explore the feasibility of the coagulation−flocculation process for sludge dewatering, the sludge was first rapidly mixed with PACl at 200 rpm for 1 min, followed by gentle mixing with BF-ADSW at 50 rpm for 9 min. Then the dewatering performance and related physicochemical properties of these sludge samples were analyzed with the same procedure as for those conditioned by PACl and BF-ADSW individually. Finally, the bound water content, sludge EPS, ζ potential, compressibility coefficient, and microstructure were tested to verify the possible enhancing mechanisms by PACl, BF-ADSW, and their combined process. The bound water content in the sludge was measured on the basis of the centrifugation method: after removal of the supernatant by centrifugation at 1000 rpm for 10 min, the water remaining within the centrifuged sludge was the bound water.24 The sludge EPSs contained soluble EPSs (S-EPSs) in the sludge supernatant and bound EPSs (B-EPSs) in the sludge pellet, whose main compounds were quantitatively validated in this study according to the method reported by Chen et al.3 The sludge samples were first centrifuged at 3000 rpm for 5 min, and the supernatant was collected as S-EPSs. Then the B-EPSs in the sludge were extracted by a heat extraction method3,25 and analyzed according to Liu et al.26 Both SEPSs and B-EPSs were analyzed in terms of polysaccharide and protein contents. The polysaccharide and protein contents were determined by the anthrone−sulfuric acid and Lowry−Folin methods, respectively.27,28 The ζ potential of the sludge was analyzed by a Zetasizer 2000 (Malvern Instruments Ltd., England) on the basis of previous studies.29 The compressibility coefficient was measured according to the methods proposed by Qi et al.30 The microstructure was characterized with environmental scanning electron microscopy (Quanta 200, FEI Ltd., The Netherlands).

MATERIALS AND METHODS

Biological Sludge and Chemicals. Biological sludge for dewatering tests was obtained in the summer from the secondary sedimentation tank of a WWTP at Wenjiang City, Sichuan Province, China. This wastewater treatment plant treats approximately 30000 m3 daily by the oxidation ditch process. The main sludge characteristics were as follows: pH value of 6.5, total sludge solids content of 14.9 g/L, volatile suspended solids (VSS) content of 6.8 g/ L, dry solids (DS) content of 12.1%, specific resistance to filtration (SRF) of 11.3 × 1012 m/kg, moisture content (MC) of 98.5%, settled volume after 30 min (SV30) of 96.1%, bound water content of 3.56 g/ g of DS, capillary suction time (CST) of 132 s, and ζ potential of −15.4 mV. PACl (commercial grade) was purchased from KeLong Chemical Reagent Co. (Chengdu, China). BF-ADSW Production. A biopolymer named “BF-ADSW” was harvested from ADSW by culturing a biopolymer-producing microorganism. The ADSW was selected from a local piggery farm at Santai City, Sichuan Province, China. The main characteristics of the ADSW were as follows: pH value of 6.7, chemical oxygen demand (COD) concentration of 1182 mg/L, ammonia concentration of 863 mg/L, and total phosphorus (TP) concentration of 28 mg/L. The biopolymer-producing microorganism, Bacillus subtilis, was a highammonia-resistant strain, enriched from biological sludge by using Luria−Bertani media (LB media) with addition of 1000 mg/L ammonia in our laboratory.20 For BF-ADSW production, B. subtilis was inoculated directly in ADSW and incubated on a reciprocal shaker at 150 rpm and 30 °C. After 60 h, the fermentation broth was collected, from which BF-ADSW was extracted by using the methods proposed by Aljuboori et al.21 Measurement of Flocculating Activity. The flocculating activity was measured in jar tests referenced to the method reported in our previous paper,13 in which 4 g/L kaolin clay suspension was

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RESULTS Production and Characteristics of the BF-ADSW. Figure 1 clearly shows that a maximum BF-ADSW yield of 3.14 g was extracted from 1 L of fermentation broth after 60 h of cultivation at the ADSW media’s initial pH point of 7.5. The B

DOI: 10.1021/acssuschemeng.8b02086 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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Figure 1. Effects of the ADSW media’s initial pH on BF-ADSW production.

corresponding flocculating activity of BF-ADSW was detrmined to be 95.8%. The BF-ADSW yield of 3.14 g/L was higher than that reported by Aljuboori et al.21 It is noteworthy that BF-ADSW can be effectively harvested by B. subtilis when the initial pH of the ADSW media was in the range of 5.5−8.5, which just covered the raw pH value of ADSW (6.7), so that pH adjustment was not needed in this experiment from a practical standpoint. At a pH point of 6.7, the BF-ADSW yield and its flocculating activity reached 2.98 g/L and 92.5%, respectively. From this conclusion, utilization of ADSW can effectively produce biopolymer and indeed reduce the production cost. Similar research reported that a natural pH was beneficial for Paenibacillus polymyxa to produce biopolymer by using potato starch wastewater, and the maximum yield was determined to be 0.81 g/L, lower than that harvested by B. subtilis from ADSW in this study.13 Chemical analysis showed that the proportion of total sugar in the BF-ADSW was 95.6% (w/w), which mainly included 50.8% neutral sugar, 26.9% uronic acid, 21.3% amino sugar, and so on, while the total protein content was 4.4% (w/w). This information was similar to that reported by Wang et al.,15 while a different biopolymer component (mainly protein) was reported by Peng et al.31 The different nutrition components and biophysical environments may account for the discrepancy in biopolymer components. Gel permeation chromatography indicated that the approximate molecular mass of the BFADSW was 4.21 × 105 Da, a relatively high molecular mass, compared to those of the biopolymers produced by Aspergillus flavus and Rhodococcus erythropoli.13,21 The higher molecular mass, the greater the number of adsorption points and the stronger the bridging ability supplied to the sludge particles.32,33 Furthermore, the ζ potential of the BF-ADSW was determined to be 13.2 mV, a positive charge that can effectively combine with a negative sludge particle, which further enhanced the sludge dewatering. BF-ADSW Alone Was Selected as a Conditioner for Sludge Dewatering. As shown in Figure 2a, for sludge with an initial pH in the range of 6.5−8.5, after being conditioned by different doses of the BF-ADSW, its dewatering was improved, which was proved by the increasing DS in the range of 14.7−19.8%. The best dewatering was attained when the pH of the sludge was 7.5, similar to our previous research conclusion.33 As shown in Figure 2b, compared to the DS of 12.1% and SRF of 11.3 × 1012 m/kg of the raw sludge, after conditioning by different doses of the BF-ADSW at a pH point

Figure 2. Effects of the initial pH of the sludge (a) and the BF-ADSW dose (b, c) on sludge dewatering.

of 7.5, DS was increased and varied in the range of 12.8−19.8% and SRF was decreased and varied in the range of 4.5 × 1012 to 10.5 × 1012 m/kg, which indicated that the sludge dewatering was improved. These results were similar to those reported by Guo et al.13 It must be noted that the best sludge dewatering was achieved when the BF-ADSW dose was 15% DS. At this optimal dose, the sludge DS and SRF were determined to be 19.8% and 4.5 × 1012 m/kg, respectively. Excessive BF-ADSW (>15% DS) prevents small flocs from growing large, due to the increasing electrostatic repulsion between the excessive BFADSW chains, thus enhancing the difficulty of sludge dewatering.34 Similarly, biopolymers produced by Bacillus species isolated from sediment samples of the Algoa Bay of the Eastern Cape Province of South Africa performed the highest C

DOI: 10.1021/acssuschemeng.8b02086 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering flocculating activity of 92.6% toward the kaolin suspension at its optimal dose of 0.3 g/L, and doses higher than 0.3 g/L decreased the flocculating activity.35 As shown in Figure 2c, after conditioning by increasing BF-ADSW (from 5% DS to 50% DS), both MC and SV30 of the conditioned sludge were decreased continuously. CST of the conditioned sludge was decreased as well, and the lowest value was achieved at the BFADSW dose of 15% DS. In fact, when the added BF-ADSW was adjusted to 15% DS, the sludge MC, SV30, and CST were decreased from the raw value of 98.5% to 85.7%, 96.1% to 80.5%, and 132 to 68 s, respectively. This information indicated that the utilization of BF-ADSW to improve the sludge dewaterability and settleability is significant.36 PACl Alone Was Selected as a Conditioner for Sludge Dewatering. The literature clearly shows that PACl works well in a wider pH range of 5.5−9.5 in sludge dewatering, and after conditioning by 4 g of PACl at the optimal pH point of 7.5, DS of the sludge (1 L) was increased from the raw value of 13.2% to 20.6% and SRF was decreased from the raw value of 11.3 × 1012 to 3.8 × 1012 m/kg.37 In this study, as shown in Figure 3a, compared to the DS of 12.1% and SRF of 11.3 ×

PACl dose was 10% DS. At this dose, the sludge DS and SRF were determined to be 22.5% and 3.9 × 1012 m/kg, respectively. A decline in DS and increase in SRF were observed when moving away from this PACl dose point. An excessive PACl dose may lead to the stabilization of the colloidal system again.37 As shown in Figure 3b, changes of MC and SV30 of the sludge conditioned by PACl displayed trends similar to those of the sludge conditions by BF-ADSW. When PACl addition was adjusted to 10% DS, the sludge MC and SV30 were decreased from the raw values of 98.5% and 96.1% to 80.8% and 74.2%, respectively. CST of the conditioned sludge decreased as well, and the lowest value of 55 s was achieved. These results indicated that PACl can also be used as an efficient conditioner for enhancing sludge dewatering, and this conclusion can be proved by our previous study.37 Combined Conditioning by PACl and BF-ADSW for Enhancing the Sludge Dewaterability. Figure 3 also shows the sludge dewatering performance by the combined coagulation and flocculation process. PACl was selected as the coagulant, and the BF-ADSW was selected as the flocculant. As shown in Figure 3a, for the sludge after treatment by PACl at different doses, DS was increased and SRF was decreased with the addition of 15% DS of BF-ADSW. For example, for the sludge after conditioning by 10% DS of PACl, further conditioning by 15% DS of BF-ADSW can increase DS from 22.5% to 28.7% and reduce SRF from 3.9 × 1012 to 2.2 × 1012 m/kg. As shown in Figure 3b, for the sludge after treatment by PACl at different doses, SV30 and MC were further decreased with the addition of 15% DS of BF-ADSW. For example, for the sludge after conditioning by 10% DS of PACl, further conditioning by 15% DS of BF-ADSW can reduce MC from 80.8% to 62.3% and SV30 from 74.2% to 49.2%. In addition, after conditioning by 10% DS of PACl and 15% DS of BFADSW, CST was decreased from the raw value of 132 to 32 s, lower than that of the sludge treated by 15% DS of BF-ADSW (68 s) or 10% DS of PACl (55 s) individually. These results indicated that the sludge dewatering can be obviously improved by PACl coagulation followed by BF-ADSW flocculation. Mechanisms for Enhancing Sludge Dewatering by the Coagulation−Flocculation Process: Bound Water Content in the Sludge. It is reported that the sludge dewatering performance is greatly dependent on the bound water content in sludge:38 the less the bound water, the better the sludge dewatering performance.39,40 Figure 4 shows the variation in bound water content for the three types of pretreatments: BF-ADSW flocculation alone, PACl coagulation alone, and the coagulation−flocculation process. It was observed that the bound water content was the smallest for the coagulation−flocculation process. The bound water contents of the conditioned sludge with individual use of PACl and BF-ADSW were 2.03 and 2.38 g/g of DS, respectively, and the bound water content was determined to be 1.32 g/g of DS for the sludge jointly conditioned with PACl and BF-ADSW. Compared to the raw value of 3.56 g/g of DS before conditioning, there was a reduction of the bound water contents of different levels, meaning that the sludge dewatering was improved more or less. These results agreed well with the sludge dewatering performance on the basis of the MCs shown in Figures 2 and 3 and showed that the coagulation− flocculation process can result in a better sludge dewatering than individual use of BF-ADSW and PACl.

Figure 3. Effects of PACl and combined conditioner on sludge dewatering: (a) DS and SRF; (b) MC, SV30, and CST.

1012 m/kg of the raw sludge, after conditioning by different doses of PACl at the pH point of 7.5, DS was increased and varied in the range of 14.2−22.5% and SRF was decreased and varied in the range of 3.9 × 1012 to 10.2 × 1012 m/kg, which indicated that the sludge dewatering was improved. It must be noted that the best sludge dewatering was achieved when the D

DOI: 10.1021/acssuschemeng.8b02086 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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Figure 6. Effect of 10% DS of PACl, 15% DS of BF-ADSW, and 10% DS of PACl + 15% DS of BF-ADSW on the protein and polysaccharide concentrations in sludge EPSs.

Figure 4. Effect of 10% DS of PACl, 15% DS of BF-ADSW, and 10% DS of PACl + 15% DS of BF-ADSW on the bound water content.

biological sludge always exists in the form of complexes.45 Thus, both protein and polysaccharide were released simultaneously after treatment by PACl alone, BF-ADSW alone, or PACl jointly with BF-ADSW. Because of the changes of protein and polysaccharide in sludge EPSs and the corresponding enhancement of sludge dewatering by PACl alone, BF-ADSW alone, or the coagulation−flocculation process, it can be concluded that EPSs are indeed an important factor that has a detrimental influence on sludge dewatering.39 Mechanisms for Enhancing Sludge Dewatering by the Coagulation−Flocculation Process: ζ Potential. It is reported that the ζ potential is one of the most critical factors proven to influence sludge dewatering.39 Generally, the sludge’s ζ potential is always negative (−15.4 mV in this study), and the electrostatic repulsion always causes sludge particles to repel each other, resulting in poor sludge dewatering performance.46 As shown in Figure 7, both PACl

Mechanisms for Enhancing Sludge Dewatering by the Coagulation−Flocculation Process: EPS Distribution and Composition in the Sludge. It has been reported in the literature that EPSs secreted by microorganisms are the major components of sludge floc matrixes.41 The main contents of sludge EPSsproteins and polysaccharideshave important impacts on sludge dewatering: EPSs as a significant fraction of sludge mass can bind a large amount of water, especially bound water.42−44 Thus, the release of EPSs is beneficial for sludge dewatering. As shown in Figure 5, more B-EPSs were released

Figure 5. Effect of 10% DS of PACl, 15% DS of BF-ADSW, and 10% DS of PACl + 15% DS of BF-ADSW on the released extracellular polymeric substance (EPS) concentrations (B-EPSs, S-EPSs, B-EPSs + S-EPSs).

from sludge conditioned with PACl alone (19.5 mg/g of VSS) or jointly conditioned with BF-ADSW (28.3 mg/g of VSS) than with use of BF-ADSW alone (12.6 mg/g of VSS), while the opposite trend was found for S-EPSs. It should also noted that, after the sludge was conditioned by the coagulation− flocculation process, the amount of released EPSs (S-EPSs + BEPSs) (32.2 mg/g of VSS) was greater than that by BF-ADSW alone (16.1 mg/g of VSS) or PACl alone (24.7 mg/g of VSS), meaning that the coagulation−flocculation process has a better effect on the release of EPSs from sludge to improve sludge dewatering.3 It is interesting to note that the proteins and polysaccharides in sludge EPSs were decreased from 4.1 and 11.4 mg/g of VSS to 2.4 and 7.7 mg/g of VSS, respectively, after the sludge was treated by PACl (10% DS), which were further decreased to 1.3 and 4.5 mg/g of VSS after further conditioning by BFADSW (15% DS) (Figure 6). Protein and polysaccharide in

Figure 7. ζ potentials for the sludge conditioned by 10% DS of PACl and 15% DS of BF-ADSW.

and BF-ADSW pretreatment modes of sludge tested imparted a positive charge to the sludge. Sludge’s ζ potential was increased with increasing individual PACl and BF-ADSW doses. For example, with a dose of 10% DS, the sludge’s ζ potential was increased to −8.3 mV when PACl was used alone, whereas −11.7 mV was achieved when 15% DS of BFADSW was used alone. This was because, after conditioning by PACl, a positive charge came from the aluminum species, charge neutralization occurred, and the sludge particles congregated more easily,33 while the bridge-aggregation ability of the BF-ADSW decreased the ζ potential (the positive charge E

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Figure 8. Images of sludge before and after conditioning by 10% DS of PACl, 15% DS of BF-ADSW, and 10% DS of PACl + 15% DS of BF-ADSW.

of the BF-ADSW can combine with the negative sludge particle, polar groups in the molecular chains of the BF-ADSW can adsorb the sludge particles, and the bridge aggregation occurs, which stabilizes the sludge flocs, and thus, decreases the ζ potential).47 In other words, adding coagulant destabilizes colloidal flocs by charge neutralization, and bridge-aggregation ability is obtained with the flocculant dose, which enhances the sludge dewatering.38 It also can be seen that the increments of the ζ potential were much greater when the PACl was used alone, indicating that the addition of PACl was superior to adding the BF-ADSW for sludge dewatering. The increments of the ζ potential during the coagulation−flocculation process (conditioned by 10% DS of PACl + 15% DS of BF-ADSW) were the largest (from −15.4 to −3.2 mV), compared to the sludge conditioned by individual PACl (from −15.4 to −8.3 mV) and BF-ADSW (from −15.4 to −11.7 mV), which showed that the enhancement of sludge dewatering by 10% DS of PACl + 15% DS of BF-ADSW was the largest. Thus, combined use of PACl and BF-ADSW can significantly improve the sludge dewatering. Mechanisms for Enhancing Sludge Dewatering by the Coagulation−Flocculation Process: Microstructure of Sludge Cakes. As shown in Figure 8, compared to raw sludge, flocs appear to aggregate with different pretreatments: 10% DS of PACl alone, 15% DS of BF-ADSW alone, and 10% DS of PACl + 15% DS of BF-ADSW. Use of coconditioning with 10% DS of PACl and 15% DS of BF-ADSW resulted in more compact and dense aggregates than use of PACl and BFADSW individually. Primary flocs were accumulated as small granules after conditioning by PACl, and the further added BFADSW helped reduce the gap between these granules through a bridging mechanism.23 As shown in Figure 9, the compressibility coefficient of the raw sludge cake was 2.01, which decreased to 0.63, 1.08, and 0.52 after conditioning by 10% DS of PACl, 15% DS of BFADSW, and their combination, respectively. The higher the

Figure 9. Compressibility coefficient of sludge cakes before and after conditioning by 10% DS of PACl, 15% DS of BF-ADSW, and 10% DS of PACl + 15% DS of BF-ADSW.

compressibility coefficient, the more difficult it was for the moisture to pass through the sludge cake during filtration and the worse the sludge dewatering.46 The decreasing compressibility coefficient means that the sludge moisture could be removed more easily, and the sludge dewaterability is enhanced.26

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DISCUSSION In this study, we investigated the dewatering performance of sludge exposed to three different pretreatments. The sludge jointly conditioned with PACl and BF-ADSW resulted in the best dewatering performance, on the basis of the changes of the sludge SV30, SRF, MC, and CST during the sludge dewatering process. The literature reported that reducing the amount of bound water within the sludge is a key step to achieve sludge deep dewatering.26,40 Sludge EPSs were considered as another factor that significantly affected sludge dewatering.6 Therefore, a preliminary mechanism that combines the physicochemical properties (bound water and F

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EPSs) of sludge was proposed to explain why the coagulation− flocculation process achieved a better dewatering performance. The reduction of bound water and EPSs (S-EPSs + B-EPSs) in the sludge was beneficial for sludge dewatering.42−44 As shown in Figures 4 and 5, the clear reductions in bound water and EPS contents were obtained for the sludge treated with PACl and BF-ADSW individually, which reflected a better sludge dewatering. Compared to those for the sludge that was conditioned by PACl or BF-ADSW alone, the effects of the coagulation−flocculation process on sludge dewatering were larger, because the reduction of bound water and EPSs was much higher. For example, after conditioning by 10% DS of PACl + 15% DS of BF-ADSW, the reduction of bound water was the most efficient (2.24 g/g of DS) and the release of sludge EPSs was the largest (32.2 mg/g of VSS). Through PACl treatment, the raw sludge floc was destroyed, and organic matter with a high molecular mass in sludge EPSs was converted into smaller organic compounds. Afterward, the sludge floc grew and aggregated by BF-ADSW flocculation, and the edge of the sludge flocs were more uniform, revealing that a more compact floc formed.48 Thus, the synergism of coagulation by PACl and flocculation by BF-ADSW was the most efficient to improve the sludge dewatering performance. These results led us to conclude that excellent dewatering performance for sludge in the coagulation−flocculation process was due to the synergistic effect of coagulation and flocculation. According to the classic theory of the double electrical layer, PACl compressed double electrical layers of sludge particles by charge neutralization, and then relatively more compact primary flocs were accumulated as small granules as a result of electrostatic attraction between the cationic electrolyte and negatively charged sludge surfaces.49 Moreover, with BF-ADSW attributing to the absorption bridging action, more stable particles adsorbed onto a BFADSW molecular chain, and they could be adsorbed simultaneously by other chains, leading to the formation of three-dimensional flocs, which further promoted sludge dewatering.37 Thus, there was an evident improvement compared to the single PACl and BF-ADSW treatment, and thus far, the combined use of PACl and BF-ADSW is feasible for enhancing the sludge dewatering performance.

Human participants or animals were not used in the study described in this paper. The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the support for this study by the National Natural Science Foundation of China (Grant No. 51508043) and the Basic Project of the Science and Technology Department of Sichuan Provincial (Grant No. 2016JY0015).

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CONCLUSION In this study, we investigated the application of a coagulation− flocculation process for sludge dewatering. After conditioning by PACl, the sludge SV30, SRF, MC, and CST were decreased, which were further decreased by further adding BF-ADSW. The contents of bound water and sludge EPSs were the key parameters for sludge dewatering performance. The mechanism is a synergistic effect of coagulation and flocculation in sludge dewatering during the coagulation−flocculation process, resulting in the most efficient reduction of bound water (2.24 g/g of DS) and the largest EPS release (32.2 mg/g of VSS). All in all, the coagulation−flocculation process is a feasible and potential process for improving the sludge dewatering performance.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*Phone/fax: +86 28 85966913. E-mail: [email protected]. ORCID

Junyuan Guo: 0000-0002-5334-8858 G

DOI: 10.1021/acssuschemeng.8b02086 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Research Article

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DOI: 10.1021/acssuschemeng.8b02086 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX