Influence of Al3+ addition on the flocculation and sedimentation of activated sludge: Comparison of single and multiple dosing patterns

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Influence of Al3þ addition on the flocculation and sedimentation of activated sludge: Comparison of single and multiple dosing patterns Yue Wen a,*, Wanlin Zheng a, Yundi Yang a,b, Asheng Cao a,c, Qi Zhou a a

State Key Laboratory of Pollution Control and Resource Reuse, College of Environmental Science and Engineering, Tongji University, Shanghai 200092, PR China b Department of Environmental Health Sciences, School of Public Health, University of Michigan, Ann Arbor 48104, MI, USA c Shanghai Municicpal Engineering Design Institute (Group) CO., LTD., Shanghai 200092, PR China

article info

abstract

Article history:

In this study, the flocculation and sedimentation performance of activated sludge (AS) with

Received 6 October 2014

single and multiple dosing of trivalent aluminum (Al3þ) were studied. The AS samples were

Received in revised form

cultivated in sequencing batch reactors at 22  C. The dosages of Al3þ were 0.00, 0.125, 0.5,

21 January 2015

1.0, and 1.5 meq/L for single dosing, and 0.1 meq/L for multiple dosing. Under single dosing

Accepted 20 February 2015

conditions, as Al3þ dosage increased, the zeta potential, total interaction energy, and

Available online 3 March 2015

effluent turbidity decreased, whereas the sludge volume index (SVI) increased, indicating that single Al3þ dosing could enhance sludge flocculation, but deteriorate sedimentation.

Keywords:

By comparison, adding an equal amount of Al3þ through multiple dosing achieved a similar

Activated sludge

reduction in turbidity, but the zeta potential was higher, while the loosely bound extra-

Flocculation

cellular polymeric substances (LB-EPS) content and SVI remarkably declined. Although the

Sedimentation

difference in the flocculation performances between the two dosing patterns was not

Interaction energy

significant, the underlying mechanisms were quite distinct: the interaction energy played a

Extracellular polymeric substances

more important role under single dosing conditions, whereas multiple dosing was more

Aluminum ion

effective in reducing the EPS content. Multiple dosing, which allows sufficient time for sludge restructuring and floc aggregation, could simultaneously optimize sludge flocculation and sedimentation. © 2015 Elsevier Ltd. All rights reserved.

1.

Introduction

The activated sludge (AS) system is the most widely used process for wastewater treatment. The effluent water quality is largely influenced by the solideliquid separation in the

system, which depends on efficient sludge flocculation and sedimentation. However, 70%e90% of the secondary clarifiers in the AS systems have encountered solideliquid separation problems (Liao et al., 2001). Reproduction of filamentous bacteria and deficient sludge flocculation are among the main causes of these problems (Jin et al., 2003). Dosing of flocculants

* Corresponding author. Room 301, Mingjing Building, College of Environmental Science and Engineering, Tongji University, Shanghai 200092, PR China. Tel./fax: þ86 21 65982697. E-mail address: [email protected] (Y. Wen). http://dx.doi.org/10.1016/j.watres.2015.02.053 0043-1354/© 2015 Elsevier Ltd. All rights reserved.

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is one of the most common solutions to poor flocculation performance, which also helps in the removal of phosphorus from wastewater, resulting in an improvement in effluent water quality. When added to wastewater, cations serve as flocculants and facilitate flocculation by causing compression of double electrical layers and ion-bridging through extracellular polymeric substances (EPS) (Sheng et al., 2010). High cation concentration could improve biological flocculation because it contributes to the continuous compression of double electrical layers (Liu et al., 2007). The Derjaguin-Landau-VerweyOverbeek (DLVO) theory elaborates the interaction energy theory in terms of colloidal particles: thinner double electrical layers tend to reduce the repulsive energy among colloidal particles. According to Liu et al. (2010), the extended DLVO theory can be applied in AS flocculation studies. EPS also have crucial influences on AS flocculation performance (Liu et al., 2007). As an important component of the AS flocs, EPS consist of polysaccharides, proteins, humic compounds, and other cellular ingredients (Sobeck and Higgins, 2002). EPS are the combination of polymers of microbial secretion, cell lysis, hydrolysis products, and organics absorbed from wastewater (Nielsen et al., 1997). The interior layer of the EPS is a tightly bound EPS (TB-EPS), which has a certain shape and adheres to the cellular surface closely and stably, whereas the exterior layer of the EPS is a loosely bound EPS (LB-EPS), which is a loose and dispersible slime layer and has no clear boundaries (Sheng et al., 2010). EPS play an important role in maintaining AS floc structure and function (Sobeck and Higgins, 2002; Sheng and Yu, 2006). In addition, they influence the physicochemical properties of biological flocs, affecting their surface charge, flocculation, sedimentation, dewatering, and absorption abilities. In the stable system of mixed microbial communities, EPS allow microorganisms to reproduce continuously at high cell densities, suggesting that the EPS matrix is a medium that enables cooperation and communication among cells in microbial aggregates (Laspidou and Rittmann, 2002). In most of the cases, dense, strong, and large flocs are desirable for AS settling and compaction (Jin et al., 2003). The suspended solids aggregate into AS flocs, which contain microorganisms as well as organic and inorganic particles embedded in the EPS (Biggs and Lant, 2000; Jin et al., 2003; Wilen et al., 2003). Previous studies have shown that the amount of EPS extracted from the AS is positively correlated to the sludge volume index (SVI) (Jin et al., 2003). Wang et al. (2013) recently reported a similar linear correlation of both LB-EPS and TB-EPS content with SVI. EPS can bind to the microbial cells by bridging with multivalent cations, which are thus likely to have an influence on the EPS content in the system (Sheng et al., 2010). An increase in monovalent cations in the AS deteriorates sludge properties and damages floc structure (Kara et al., 2008), while addition of multivalent cations is an effective way to improve sludge flocculation (Higgins et al., 2004). Bruus et al. (1992) found that addition of divalent copper can maintain the three-dimensional structure of EPS, thereby improving sludge flocculation. Furthermore, Park et al. (2006) and Li et al. (2012) observed that, compared to monovalent and bivalent cations, trivalent cations contribute to better flocculation performance

of the AS under both wastewater treatment plant (WWTP) and laboratory conditions. This may be owing to the fact that trivalent cations such as those of aluminum (Al3þ) and iron (Fe3þ) possess stronger abilities to bind to the sludge matrix and can therefore enhance sludge floc stability owing to their higher charge valence and lower solubility (Kakii et al., 1985; Abu-Orf et al., 2004). Both Al3þ and Fe3þ, which are widely adopted as coagulating agents in WWTP, can be utilized to neutralize the sludge surface charge and promote AS flocculation and sedimentation performance (Higgins and Novak, 1997; Subramanian et al., 2010). However, Li et al. (2012) reported that Al3þ dosing resulted in the formation of larger flocs, when compared with Fe3þ dosing. From the above-mentioned findings, the advantage associated with Al3þ dosing over the use of other cations in terms of promoting sludge flocculation can be recognized. However, the dosing pattern, which is a significant concern for engineering applications, has not yet been systematically studied, and there are only limited results on the influence of the dosing pattern on AS flocculation and sedimentation, with the underlying mechanisms still being unclear. In the present study, Al3þ was added to AS systems according to single and multiple dosing patterns, and a concentration gradient was set under single dosing conditions. The research objectives were as follows: (1) to investigate the influence of Al3þ dosage on AS flocculation and sedimentation; (2) to determine the more efficient dosing pattern; and (3) to reveal the mechanisms governing the influence of Al3þ on AS flocculation and sedimentation.

2.

Materials and methods

2.1.

AS cultivation

The AS used in this study was cultivated in two parallel sequencing batch reactors (SBRs) numbered ReS (single dosing) and R-M (multiple dosing), and each had a working volume of 4 L. The reactors were seeded with AS (approximately 2500 mg/L) from Qu Yang WWTP in Shanghai, China. Each reactor was equipped with a paddle mixer operating at 100 rad/min to prevent the AS from settling; air was introduced to maintain the dissolved oxygen (DO) in the range of 2e3 mg/L; sludge retention time (SRT) was controlled around 10 d; and temperatures in the reactors were maintained at 22 ± 1  C. The SBRs were operated at a cycle time of 12 h, and 2 cycles were performed each day. The time used for filling, aerating, settling and decanting was 20, 640, 40 and 20 min respectively. The volumes of both the influent and effluent per cycle were maintained at 2 L. Tap water (containing ~0.007 meq/L of Al3þ) was used to synthesize the artificial wastewater. Glucose, NH4Cl and KH2PO4 were used as sources of carbon (C), nitrogen (N) and phosphorus (P) respectively, and the relative proportion of chemical oxygen demand (COD), N and P (COD: N: P ratio) in the influent was maintained at 100: 5: 1, with the COD concentration at 700 ± 30 mg/L. Micronutrient concentrations were maintained according to the protocol given by Liao et al. (2011), and NaHCO3 was added to control the influent pH in the range of 6.8e7.2.

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2.2.

Al3þ dosing test

2.2.1.

Single dosing test

The AS in reactor ReS was cultivated without adding Al3þ in the influent. After the reactor operation stabilized, 500 mL of AS sample were decanted into 5 identical 250 mL beakers, each receiving 100 mL, to which a different Al3þ dosage was added. The Al3þ dosages were 0.125, 0.5, 1 and 1.5 meq/L, respectively, in beakers designated as S-0.125, S-0.5, S-1.0, S1.5, respectively. No Al3þ was added into beaker S-0, which was the blank control of the experiment. All of the Al3þ in the single dosing test was added once. The samples were then mixed at 117 rad/min for 5 min and subsequently at 50 rad/ min for 5 min.

2.2.2.

EPS extraction protocol

The sample extraction protocol adopted in this study was based on the modification of previous research methods (Li and Yang, 2007; Morgan et al., 1990; Yu et al., 2009). A 25-mL sample of AS suspension was centrifuged at 6000 g for 10 min at 4  C, and the supernatant that was carefully collected was bulk solution. A NaCl solution with the same conductivity as the AS sample was prepared; then it was utilized to re-suspend the AS material in the tube at its original volume before the bulk solution was extracted. With no delay, the AS suspension was oscillated by a vortex mixer (S25, IKA, Germany) for 1 min and then sheared using an ultrasonication instrument (Digital Sonifier 450, Branson, USA) at 0.4 W/mL for 30 s. Subsequently, the suspension was centrifuged at 4000 g for 10 min at 4  C, and the supernatant was collected as LB-EPS. The AS sample left in the tube was resuspended to its original volume of 25 mL with the NaCl solution, and then oscillated for 1 min and sheared at 0.5 W/mL for 1 min. Subsequently, the suspended sample was centrifuged at 4000 g for 15 min at 4  C, from which the supernatant collected was TB-EPS, and the AS sample left in the tube was the pellet. As a final step, the bulk solution, LB-EPS and TB-EPS extracted were filtrated through a 0.45 mm cellulose acetate membrane.

2.4.

measuring the SVI. The turbidity of the supernatant was measured using a turbidity meter (2100P, HACH, United States); the contact angle was determined using a contact angle analyzer (JC2000D, Powereach Co., Shanghai, China); and the zeta potential of supernatant flocs was quantified using a Zetasizer (Nano Z, Malvern, United Kingdom). The total interaction energy of AS was measured and calculated referring to the approach introduced by Liu et al. (2007).

3.

Results and discussion

3.1.

Basic characteristics of the AS systems

Multiple dosing test

In the multiple dosing test, Al3þ at a concentration 0.1 meq/L was added to the influent of reactor R-M and introduced to the reactor in each operation cycle. The mixing pattern was in accordance with single dosing test.

2.3.

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Analytical techniques

The total organic carbon (TOC) contents of the bulk solution, LB-EPS and TB-EPS were determined using a TOC/TN analyzer (TMM-1, SHIMADZU, Japan). The Al3þ concentrations in the bulk solution, LB-EPS, TB-EPS and pellet were analyzed using inductively coupled plasma optical emission spectrometry (ICP-AES, Optima 2100 DV, Perkin Elmer, United States). Aqua regia digestion was employed to digest the pellet following the standard method (USEPA, 1998) before measuring its cation concentrations. After settling for 30 min, 100 mL of AS suspension were suctioned and transferred into a graduated cylinder for

The influent and effluent characteristics of ReS and R-M are listed in Table S1. Very similar AS concentrations (expressed as mixed liquor volatile suspended solid) were observed in the two reactors (Table S1) and the same SRT of 10 d was maintained, suggesting that there was only a marginal difference in sludge production (quantified as excess sludge amount) between the two reactors. Furthermore, since the daily influent volume was the same as the working volume of the reactor and the SRT was controlled at 10 d, the amount of Al3þ dosed to per unit of AS mass at a concentration of 0.1 meq/L through multiple dosing was equal to that added at a concentration of 1 meq/L though single dosing. This indicated that despite the difference in the dosing patterns, the same dosage of Al3þ was added to the AS in samples S-1.0 and M-0.1. The turbidity of the control sample (S-0) was as high as 10.4 ± 0.14 NTU (Table 1). As Al3þ in the single dosing experiment was sequentially added, the turbidity of the supernatant in these samples declined as the dosage increased, reaching as low as 2.03 ± 0.46 NTU (S-1.5), which signified that AS flocculation could be promoted by single Al3þ dosing. Moreover, as the Al3þ dosage increased, the SVI tended to increase, with a sharp increase occurring from S-0.5 to S-1.0 (Table 1). This finding suggested that under single dosing conditions, AS settling and compressing performances deteriorated, whereas flocculation improved. In the multiple dosing scenario, the turbidity of M-0.1 (5.29 ± 0.54 NTU) was comparable to that of S-1.0 (3.51 ± 0.12 NTU), but was remarkably lower compared with that of M0 (10.4 ± 0.14 NTU). Thus, when an equivalent amount of Al3þ was added, AS flocculation was promoted via both of the dosing patterns, and the difference in enhancement was negligible. However, the SVI reached its lowest level (38 ± 1 mL/g) in the multiple dosing reactor M-0.1, and much higher SVI values were observed both when an equivalent dosage of Al3þ was added through single dosing (sample S1.0, 123.7 ± 25 mL/g) and when no Al3þ was added (sample S-0, 90.7 ± 21 mL/g). Thus, multiple Al3þ dosing can improve AS flocculation and sedimentation simultaneously. The possible explanations for the observed enhancement in AS flocculation following Al3þ dosing could be the compression of the double electrical layers and/or decrease in the EPS; these possibilities will be discussed in the following sections.

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Table 1 e The characteristics of AS flocculation and sedimentation. Test Al3þ dosage Supernatant (meq/L) turbidity number (NTU) Single dosing

Multiple dosing

S-0 S-0.125 S-0.5 S-1.0 S-1.5 M-0.1

0.00 0.125 0.5 1.0 1.5 0.1

10.4 7.31 7.22 3.51 2.03 5.29

± 0.14 ± 2.09 ± 0.69 ± 0.12 ± 0.46 ± 0.54

SVI (mL/g) 90.7 ± 100.1 ± 97.2 ± 123.7 ± 132.5 ± 38 ±

21 19 23 25 29 1

Contact angle ( ) 19.1 20.3 21.6 21.5 22.9 19.9

Zeta potential (mV)

± 0.74 14 ± 0.58 9.37 ± 1.29 7.06 ± 1.31 4.14 ± 2.44 2.60 ± 1.0 10.35

± 2.43 ± 1.32 ± 0.62 ± 1.03 ± 0.87 ± 1.05

Bulk solution LB-EPS content TB-EPS content content (mg (mg TOC/g VSS) (mg TOC/g VSS) TOC/g VSS) 2.84 3.06 2.49 2.12 2.21 1.97

± 0.23 ± 0.24 ± 0.24 ± 0.22 ± 0.23 ± 0.02

4.86 5.29 4.36 3.24 2.60 0.89

± 0.39 ± 0.40 ± 0.38 ± 0.33 ± 0.27 ± 0.10

8.63 ± 8.35 ± 7.64 ± 7.44 ± 7.35 ± 2.04 ±

0.99 0.92 0.80 0.80 0.69 0.38

Note: All the value of parameters were showed as: mean ± one standard deviation (n ¼ 5).

3.2.

Influence of Al3þ on AS interaction energy

3.2.1.

Single dosing

The total interaction energy curves of the single dosing test indicated that the energy barrier decreased as the Al3þ concentration increased (Figure S1). When the Al3þ dosage exceeded 1 meq/L, the energy barrier disappeared and the total interaction energy became negative. The turbidities of all single-dose samples followed the same decreasing tendency as that of the energy barriers (Table 1). According to the DLVO theory, accumulation of negative surface charges could lead to an increase in repulsive electrostatic interactions between the approaching surfaces and weaken the bonding between the flocs (Zita and Hermansson, 1994; Wilen et al., 2003). As Al3þ can neutralize the negative charges on the surface of the sludge particles and reduce energy barriers, high Al3þ dosages are favorable in terms of particle bonding and aggregate formation. The effluent turbidity declined with the decreasing number of suspended particles, which explains the decrease in turbidity associated with the increase in Al3þ dosage. In S-1.0, the zeta potential reached the lowest level at 4.14 ± 1.03 mV and the energy barrier disappeared, which limited the quantitative analysis. However, in this case, the zeta potential could be adopted as an alternative to the Stern potential, which is the most significant parameter that affects the interaction energy (Liu et al., 2007). The effects of other parameters such as contact angle on interaction energy were excluded because they showed minimal differences in magnitude (Table 1). Thus, the changes in the AS interaction energy in response to Al3þ dosages and dosing patterns can be represented by the variations in zeta potential. The absolute value of zeta potential in the single dosing test was negatively correlated to the Al3þ dosage (R2 ¼ 0.83), while the supernatant turbidity showed a consistent trend with the Al3þ dosage (R2 ¼ 0.99) (Fig. 1). This observation can be explained by the double electrical layer theory. The colloids that carried the positive charges, formed as a result of Al3þ hydrolysis, compressed the double electrical layers effectively and reduced the zeta potential on the particle surfaces. Subsequently, the particles destabilized and aggregated into AS flocs, lowering the effluent turbidity.

3.2.2. Comparison between single and multiple dosing patterns The absolute value of the zeta potential of M-0.1 was lower than that of S-0 (Table 1), indicating that multiple Al3þ dosing

also compressed the double electrical layers through charge neutralization; however, the zeta potential reduction was not as dramatic as that observed when an equivalent dosage of Al3þ was added in the single dosing test (S-1.0). In particular, when compared with S-0, the zeta potentials of S-1.0 and M0.1 decreased by 70.4% and 26.1%, respectively, whereas the turbidity of S-1.0 and M-0.1 decreased by 66.2% and 49.1%, respectively. Although the reductions in the turbidity were quite similar, the decrease in zeta potential achieved by single Al3þ dosing was markedly higher than that observed following multiple Al3þ dosing. This implied that, in addition to zeta potential, other factors also had an effect on AS flocculation, which compensated for the relatively weak zeta potential reduction following multiple Al3þ dosing and ultimately equilibrated the effluent turbidities. However, a divergence was observed when only dosing concentration was considered; while the decrease in turbidity of M-0.1 (49.1% of S-0) was higher than that of S-0.125 (29.7%), the reduction in zeta potential was lower (Table 1). As zeta potential represents the surface rather than the interior charge of the sludge, the absolute value of zeta potential should decline with the increasing Al3þ concentration, regardless of the dosing pattern. Thus, the observed similar decrease in the zeta potentials of S-0.125 and M-0.1 was the result of their comparable Al3þ dosages. Furthermore, it is clear that other factors contributed to the lower turbidity

Fig. 1 e The relations among Al3þ dosage, zeta potential of supernatant flocs and supernatant turbidity. Error bars represent mean values ± one standard deviation.

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related to multiple Al3þ dosing. These observations suggested that the investigation should not be restricted to zeta potential only, and that the total interaction energy should also be taken into consideration. The sharp decrease in the total interaction energy of M-0.1, when compared with that of S-0 (Figure S1), indicated that multiple Al3þ dosing can promote AS flocculation by compressing the double electrical layers. The energy barrier disappeared in the single dosing test, while it remained at 397 kT in the multiple dosing test at an equivalent Al3þ dosage; however, despite the difference in the energy barrier of these samples, their supernatant turbidities were comparable. Furthermore, when compared with S-0.125, M-0.1 exhibited a higher energy barrier and lower turbidity (Figure S1), which suggested the difficulty in clarifying how dosing pattern affects flocculation performance in terms of total interaction energy. As the classical DLVO theory is based only on the interactions between two particles suspended in a liquid, as a supplement, the Lewis acidebase interaction was included in the extended DLVO theory (Wu and Nancollas, 1999; Liu et al., 2007). Depending on the components of surface tension, parameters of dispersed surfaces, and dispersing medium, the Lewis acidebase interaction can result in either an attractive or repulsive force among particles dispersed in a polar liquid (Azeredo et al., 1999; Wu and Nancollas, 1999). While both the classical and extended DLVO theories are based on the assumption that particles have smooth surfaces and are uniform in shape, in reality, the surface properties of the AS particles are extremely complicated owing to the existence of EPS. Thus, to evaluate the influence of Al3þ on AS flocculation, the role of EPS, besides that of interaction energy, was carefully examined.

3.3.

Influence of Al3þ on the EPS content

3.3.1.

Single dosing test

Wilen et al. (2008) found a positive correlation between effluent turbidity and EPS content in WWTP with Fe3þ dosing, which revealed that high EPS content could worsen sludge flocculation. As EPS can be divided into LB-EPS and TB-EPS, it is reasonable to further investigate which one of these two (or both) actually affects sludge flocculation. As shown in Table 1, the variation in the organic content in the bulk solution of different sludge samples was minimal, but the content of LBEPS and TB-EPS tended to decrease as the single dosage increased, forming a clear linear correlation between Al3þ concentration and LB-EPS (R2 ¼ 0.95) and a relatively weaker correlation between Al3þ concentration and TB-EPS (R2 ¼ 0.74). Besides, LB-EPS showed more evident variation in content than TB-EPS, because TB-EPS are located in the interior layer of the sludge, and are thus less exposed to the external environment (where the addition of Al3þ occurs) that directly affects LB-EPS. Moreover, LB-EPS have loose spatial structures that can be compressed easily; previous studies have also shown that sludge flocs become denser and more compact with Al3þ addition (Sheng et al., 2006; Li et al., 2012). Li and Yang (2007) found that LB-EPS play an important role in the formation of AS flocs; however, an excess of LB-EPS might have a negative effect on the bonding of cells and could deteriorate the floc structure, eventually causing

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undesirable solideliquid separation. In the present study, the LB-EPS and supernatant turbidity exhibited a similar tendency to decrease as Al3þ dosage increased (Table 1), which further proved the finding of Li and Yang (2007). As LB-EPS contain a considerable amount of bound water, a high LB-EPS content may result in high water content in the sludge flocs, leading to worsened sludge compactness and increased floc porosity, ultimately causing poor flocculation (Poxon and Darby, 1997; Li and Yang, 2007). Meanwhile, the loose structure of LB-EPS could negatively affect the bonding of particles that occurs on the sludge surface; thus, the superabundance of LB-EPS may lead to an increase in suspended particles in water and an increase in effluent turbidity. The addition of Al3 was found to ameliorate AS flocculation by counteracting the negative effect of LB-EPS. With relatively high valence and polarization, Al3þ barely gets separated from the sludge once combined, and with the binding of Al3þ to the sludge, the sludge density is enhanced and the extraction of LB-EPS becomes difficult. Furthermore, Al3þ hydrolyzes in water and produces hydroxyl complexes that bridge with each other and form polymeric networks that seize the suspended small particles, eventually reducing the suspended solids in the supernatant and improving effluent turbidity.

3.3.2.

Comparison between single and multiple dosing

According to Sobeck and Higgins (2002), the bridging function of bivalent cations could well explain the long-term effect of ions on the transformation of aggregates. However, our previous research indicated that the influence of trivalent cations on the LB-EPS content and sludge flocculation is more significant than that of bivalent cations (Li et al., 2012). In the present study, following the addition of an equivalent Al3þ dosage, both the LB-EPS and TB-EPS contents extracted in the multiple dosing test were significantly lower than those extracted in the single dosing test (Table 1). Therefore, in the multiple dosing test, Al3þ effectively enhanced sludge densification through the ion bridging effect. As explained in Section 3.2.2, the discussion regarding the influence of dosing patterns on sludge flocculation cannot be restricted to interaction energy alone, and taking EPS into consideration could shed some light on the analysis. In other words, in the equivalent dose scenario, similar flocculation enhancements were achieved by single and multiple dosing patterns because of the combined effect of EPS content and interaction energy. Single Al3þ dosing exhibited a greater effect on the interaction energy, while multiple Al3þ dosing more effectively reduced the EPS content. The advantage of single Al3þ dosing in manipulating interaction energy was counterbalanced by its weakness in decreasing the EPS content, and vice versa for multiple dosing. Thus, the eventual flocculation performances achieved by the two dosing patterns were equal. In the following section, the effect of different dosing patterns on the interaction energy and EPS content in the AS will be discussed.

3.4.

Al3þ distribution characteristics in the AS

3.4.1.

Distribution characteristics

With the increasing single dosage, the Al3þ concentration detected in the sludge gradually increased (Fig. 2). Al3þ in the

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-16

8

-14

Pellet TB-EPS LB-EPS Bulk Solution

6 5

Zeta potential of supernatant flocs (mV)

Al3+ concentration (mg/g VSS)

7

4 3 2 1

Single dosing

-12

Multiple dosing

-10 -8 -6 -4 R2=0.76

-2 0 7

0

0.5

1

1.5

0.1

Single

Single

Single

Multiple

Al dosage (meq/L) Fig. 2 e The distribution of Al3þ in different AS components in single and multiple dosing tests.

sludge of S-0 originated from the influent. Owing to its high charge valence and low solubility (Kakii et al., 1985; Abu-Orf et al., 2004), almost all Al3þ dosed bound with the sludge. Thus, no Al3þ could be detected in the bulk solution. Liu et al. (2010) found that the energy barrier of the total interaction energy of aerobic sludge increased subsequently after the extraction of LB-EPS and TB-EPS. Because the Stern potential is the dominant parameter that affects the interaction energy as discussed in Section 3.2.1, the finding above indicated that the amount of negative charges in different sludge components followed the sequence: pellet > TBEPS > LB-EPS. In other words, the pellet was the most negatively charged sludge component and provided the most bonding sites for Al3þ. Besides, EPS bind with cells through complex interactions to form a vast net-like structure (Sheng et al., 2010). Thus, Al3þ was able to pass through LB-EPS and TB-EPS successively, and combine with the pellet of the sludge directly. Therefore, no Al3þ could be detected in sludge components other than the pellet when the single dosage was less than 0.125 meq/L. Further, as the dosage increased, the accumulation of Al3þ in TB-EPS was higher than that in LBEPS, but the majority of Al3þ was always found in the pellet (Fig. 2). When an equivalent amount of Al3þ was added through single and multiple dosing, similar Al3þ accumulations were noted in the sludge, but the distributions of Al3þ among the various sludge components were different (Fig. 2). Al3þ could be detected in the bulk solution in the multiple dosing test, but not in the single dosing test. Furthermore, Al3þ accumulation in the pellet was slightly higher in the multiple dosing test, when compared with that in the single dosing test. These findings indicated that the bonding capacity between Al3þ and the sludge varied with the dosing pattern.

Single dosing

As shown in Fig. 3, the results of the single dosing test pointed to the negative correlations between the pellet Al3þ concentration and the absolute value of zeta potential, LB-EPS

(b) 6

LB-EPS content (mg TOC/g VSS)

0.125 Single

Single dosing Multiple dosing

5 4 R2=0.92

3 2 1 0 12

(c) Single dosing

10

Supernatant turbidity (NTU)

0 Single

3+

3.4.2.

(a)

Multiple dosing

8 6 4

2

R =0.94

2 0 0

1

2

3

4

5

6

7

The Al3+ concentration in the pellet (mg/g VSS)

Fig. 3 e The influence of Al3þ concentration in the pellet of AS on (a) zeta potential of supernatant flocs, (b) LB-EPS content and (c) supernatant turbidity in single and multiple dosing tests. Error bars represent mean values ± one standard deviation (n ¼ 5).

content, and supernatant turbidity (R2 ¼ 0.76, 0.92, and 0.94, respectively). Ideal correlations were observed between Al3þ dosage and both LB-EPS content and supernatant turbidity. As the pellet is at the core of the sludge, the Al3þ concentration in the pellet was better correlated to the LB-EPS content than was the zeta potential, reflecting the charges on the surface of the sludge. Hence, as the single Al3þ dosage increased, the amount of Al3þ in the pellet also increased, which resulted in an increase in the density of the sludge flocs (Figure S3) and a decrease in the content of suspended solids, leading to improved effluent turbidity.

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3.5.

Influence of Al3þ on AS settleability

3.5.1.

Single dosing

Most researchers believe that the SVI increases with increasing sludge EPS content, indicating poor sedimentation (Liu and Fang, 2003). However, in the present study, the SVI value was negatively correlated to the LB-EPS content in the single dosing test (R2 ¼ 0.86) (Fig. 4). It has been reported that a higher single dosage of flocculant could increase the nucleation rate and the amount of small aggregates (Gonzalez et al., 2007), and strengthen the interaction energy among the organics on the sludge surface through the bridging function (exhibited as EPS reduction). Nevertheless, in the single dosing scenario, the reaction time was not sufficient for the formation of large aggregates, and as small flocs took more time to settle, the SVI value of the AS increased as the Al3þ dosage increased, indicating an undesirable sedimentation performance (Gonzalez et al., 2007).

3.5.2.

6

Comparison between single and multiple dosing

Following the addition of equivalent Al3þ dosages, the absolute value of zeta potential generated by multiple dosing was markedly higher than that achieved by single dosing (Fig. 3(a)). A contrasting result was observed with respect to LB-EPS content (Fig. 3(b)), and no substantial difference was noted with regard to supernatant turbidity (Fig. 3(c)). Therefore, it can be concluded that pellet Al3þ concentration had an essential effect on sludge flocculation. The binding of Al3þ to the sludge pellet promoted compression of the double electrical layers, facilitated reduction in the sludge surface charge, decreased the LB-EPS content, and maintained sludge structure, thus resulting in improved sludge flocculation. Li et al. (2014) suggested that when multivalent cations are added to AS systems, an extension of SRT could decrease the LB-EPS content and promote sludge flocculation. In other words, the time for the reaction between multivalent cations and the AS is critical to flocculation performance. When 1 meq/L of Al3þ was added in a single dosage, the bonding between Al3þ and LB-EPS during the limited reaction time was not sufficient, and thus more Al3þ remained on the sludge surface, as compared to reaction time during the multiple dosing test (Fig. 3), which decreased the total interaction energy (Figure S1). However, when an equivalent dosage was added multiple times, Al3þ had sufficient time to react with the sludge and travel to the pellet, and as a result, a considerable decrease in the LB-EPS content was noted. Nevertheless, the decrease in the sludge surface charges diminished due to the increased Al3þ accumulation in the pellet. Therefore, depending on the dosing pattern, the contributions of Al3þ in lowering the interaction energy barrier and decreasing the LB-EPS content were different; conversely, when the effects of the two processes were combined, the eventual flocculation enhancement observed in the single and multiple dosing systems was similar (Table 1).

Comparison between single and multiple dosing

The correlation observed in the present study between SVI and LB-EPS under single dosing conditions (R2 ¼ 0.86) is in good agreement with the findings of previous studies, indicating

Single dosing

LB-EPS content (mg TOC/g VSS)

3.4.3.

Multiple dosing

5

4

R2=0.86

3

2

1

0

40

60

80

100

120

140

160

SVI (mL/g) Fig. 4 e eThe relation between the LB-EPS content and SVI. Error bars represent mean values ± one standard deviation (n ¼ 5). that EPS content has a significant effect on sludge settleability (Ye et al., 2011). However, the results of the multiple dosing test deviated from the tendency exhibited in the single dosing test (Fig. 4). Nevertheless, with the equivalent Al3þ dosage, both the LB-EPS content and SVI observed in the multiple dosing test were lower compared with those observed in the single dosing test, suggesting satisfactory sludge settleability. Furthermore, as discussed earlier, both sludge flocculation and sedimentation were enhanced by multiple additions of Al3þ; having sufficient time to react with AS, Al3þ not only facilitated the aggregation of small flocs into larger ones, but also achieved floc densification by constantly reorganizing the sludge structure and removing water (Figure S3). In summary, the key difference between the two dosing patterns was the variation in the reaction time, which further affected the microcosmic and macrocosmic spatial characteristics of the AS, as indicated by the reduction in the LB-EPS content and decrease in the SVI value, respectively. The microcosmic effect occurred in a short time, but the realization of the macrocosmic effect required a long reaction time; as a result, the SVI value first increased, but subsequently decreased over time. Thus, with the advantage of a relatively long reaction time, multiple dosing effectively reduced both LB-EPS and SVI, and thereby promoted sludge flocculation and sedimentation simultaneously. On the contrary, owing to the short reaction time in single dosing conditions, augmentation and densification of sludge flocs could not be accomplished, which also explains the negative correlation between LB-EPS content and SVI in the single dosing test.

3.6.

Engineering significance

In the event of AS flocculation and/or sedimentation failure in WWTPs, a common approach to address this problem is to add flocculants once only. In the present study, single dosing simulated the approach usually taken by WWTPs, and solved the flocculation problem at the cost of deteriorating sludge settleability. Conversely, multiple dosing, which resembles

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continuous addition of flocculants in WWTPs, technically extended the reaction time and simultaneously promoted flocculation and sedimentation performance. Thus, as a practical operational procedure, it is suggested that the concentrations of multivalent cations in the pellet should be regularly monitored, and once these concentrations tend to decrease, low concentrations of flocculants should be continuously added to maintain sludge flocculation and sedimentation.

4.

Conclusions

The present study examined the effects of single and multiple Al3þ dosing patterns on sludge flocculation and sedimentation. The main conclusions are: i) Under single dosing conditions, sludge flocculation and turbidity removal improved with the increasing single dosage, but sludge sedimentation deteriorated. ii) Although there was no significant difference in turbidity reduction between single and multiple dosing patterns at an equivalent Al3þ dosage, better sludge sedimentation was achieved with multiple dosing, as indicated by the dramatic decrease in the SVI value. iii) Single dosing was more effective in terms of reducing the interaction energy barrier, while multiple dosing was more effective in decreasing the EPS content. Furthermore, multiple dosing allowed sufficient time for Al3þ to react with sludge and promote AS flocculation and sedimentation.

Acknowledgments This work was supported by the Major Science and Technology Program for Water Pollution Control and Treatment of China (2012ZX07313-001). We thank three anonymous reviewers for helpful comments and suggestions that further improved the quality of the manuscript.

Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.watres.2015.02.053.

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