Effects of tannery sludge incineration slag pretreatment on sludge dewaterability

Chemical Engineering Journal 221 (2013) 1–7

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Effects of tannery sludge incineration slag pretreatment on sludge dewaterability Xun-an Ning ⇑, Haijian Luo, Xiujuan Liang, Meiqing Lin, Xin Liang School of Environmental Science and Engineering, Guangdong University of Technology, Guangzhou 510006, 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

" First scientific attempt to apply

The high compressibility of flocculated sludge results in sludge cake particles being deformed during the compression stage. The reduction of sludge filterability is due to the cake voids closed and, as a result, the formation of a layer of low permeability on the filter medium. However, in the case of flocculated TSISsludge, the sludge cake can form a permeable and more rigid lattice structures which prevent the formation of an impermeable layer of sludge.

sludge incineration slag to sludge dewatering. " Combine TSIS with CPAM present great advantages over CPAM conditioning alone. " TSIS conditioned can destroy the surface electric double layers structure of sludge. " TSIS aided to build incompressible and porous structure during mechanical dewatering.

a r t i c l e

i n f o

Article history: Received 24 September 2012 Received in revised form 9 January 2013 Accepted 30 January 2013 Available online 13 February 2013 Keywords: Tannery sludge Sludge incineration slag Dewaterability Compressibility Porosity

a b s t r a c t Chemical conditioning pretreatment does not always improve sludge dewaterability sufficiently for flocculated sludges with high compressibilities. In this study, tannery sludge incineration slag (TSIS) was used as a skeleton builder, in combination with cationic polyacrylamide (CPAM), to condition tannery sludge. The results showed that pretreating the sludge with a combination of TSIS and CPAM considerably improved sludge dewaterability over CPAM conditioning alone. The optimum TSIS and CPAM dosages were 150% dry solid (DS) and 10 kg/t DS, respectively, which generated a maximal net sludge solids yield (10.3 kg/m2 h) and a minimal time to filter (7 s). The negative charge of the sludge particles was neutralized by the positive charge of the dissolved metal ions of the TSIS, resulting in larger floc sizes. Compressibility and porosity measurements and scanning electron microscope (SEM) images indicated that the TSIS formed a porous and incompressible structure during mechanical dewatering. Ó 2013 Elsevier B.V. All rights reserved.

1. Introduction Wastewater treatment processes produce large amounts of sludge that commonly contain over 95% water [1]. The most important part of sludge treatment prior to disposal is the ⇑ Corresponding author. Tel.: +86 20 3932 2546. E-mail address: [email protected] (X.-a. Ning). 1385-8947/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cej.2013.01.106

reduction of the sludge volume by solid–liquid separation. The presence of organic components, mainly bacterial cells and extracellular polymeric substances (EPSs), and supracolloidal (>1 lm) particles in the sewage sludge makes the dewatering difficult even at the high pressures of mechanical dewatering [2]. Chemical conditioners, such as polyacrylamide, and surfactants are widely used to improve the mechanical dewaterability of the waste sludge, whereby flocculating sludge particles are flocculated to

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successfully decrease the filter cake water content. Particle–particle bridging and surface charge neutralization are two widely accepted flocculation mechanisms [3]. However, there is no chemical conditioner that can dewater sludge to the desired level at an acceptable conditioning dosage [4]. Polymer conditioning of sludge appears mainly affect the rate of water release and not the extent of dewatering [5]. Sorensen et al. [6] reported that polymers can adversely affect sludge was by increasing the compressibility of the filter cake. After a highly compressible sludge cake has formed in the filter, the sludge particles deform easily under pressure during the compression stage, causing cake void closure that impedes further dewatering. Accordingly, longer compression times or higher pressures are required to achieve a high solids content, compromising the efficiency of the whole treatment process. The rate of flocculated sludge dewatering in mechanical filtration can be further increased by reducing the filter cake compressibility and increasing the cake porosity [2]. Physical conditioners, known as skeleton builders or filtration aids, have been employed to improve the characteristics of the sludge cake. These physical conditioners can form permeable and more rigid lattice structures that can remain porous during mechanical dewatering [7]. Carbonbased materials have been used as skeleton builders, including lignite [2], char and fine-coal [8], and organic waste solids such as bagasse [6], wood chips and wheat dregs [9]. Inert materials have also been used, including fly ash from municipal sludge incineration [10], coal fly ash modified by sulfuric acid [11], cement kiln dust [12], and gypsum [5]. Most physical conditioners have been used with coagulants or flocculants to condition sludge. Zhao [5] found that the interaction between a polyelectrolyte and gypsum formed more compact sludge flocs resulting in higher equilibrium moisture removal. Thapa et al. [2] similarly reported the interaction of a polyelectrolyte with digested sewage sludge and lignite in sludge dewatering. Smollen and Kafaar [13] used the paralyzed domestic refuse (char) in conjunction with a polyelectrolyte to improve sludge dewatering. The results mentioned above illustrate that the addition of physical conditioners can provide a porous, permeable and rigid lattice structure for the loose flakes formed by flocculation. Sludge incineration slag is the residual waste from sludge incineration processes. Interest in sludge incineration is increasing because this process can reduce the volume and the mass of the waste by 90% and 70%, respectively [14]. Some studies have reported that sludge incineration residues have been reused for agricultural land [15], cement production [16], and as a sub-base or asphalt mixture in road construction [17]. However, the presence of inorganic salts (chlorides and sulfates) and heavy metals (copper, zinc, and lead) in the sludge incineration slag restrict its use in the applications mentioned above [18]. Therefore, more efficient sludge incineration slag utilization should be explored. Although inert materials have been previously investigated as sludge conditioners, TSIS has not been used as a skeleton builder to condition sludge. This study explored the improvement of tannery sludge dewaterability by co-conditioning with TSIS and CPAM; the optimal conditions for conditioning and dewatering and related mechanisms were also investigated. This work may provide insight into changing sludge properties and presents a new approach to sewage sludge conditioning and sludge incineration slag utilization.

sisted of a homogeneous mixture of coagulated sedimentation sludge and excess activated sludge that were first gravity thickened to approximately 98% w/w moisture content. To minimize microbial activity, the samples were stored in a refrigerator (at 4 °C for less than 14 d). The sludge sample characteristics are shown in Table 1. The mixed tannery sludge was first compressed by belt presses in the plant, after which the sludge was taken to the laboratory to be air dried and milled. The TSIS was a residue from mixed sludge combustion using a pulverized coal fired furnace with a tail gas temperature of 900 °C. A cationic polyacrylamide (FO4190SH, SNF Floerger), with a molecular weight of (5–10)  105 and a charge density of 20– 30%, was used as a sludge conditioner. The CPAM solutions (1000 mg/l) were prepared by completely dissolving the powdered polyacrylamide in distilled water, followed by ageing for 6 h prior to use. 2.2. Experimental procedure Different TSIS dosages (expressed as a percentage of the tannery sludge solids) were first added to a 500-ml sludge sample in a 1-l beaker. After several seconds of rapid mixing to ensure dispersion, the CPAM was added at dosages in the 0–20 mg/l range. To promote flocculation, the stirring speed for sludge flocculation is 300 rpm for the first 20 s, followed by 60 rpm for a further 40 s. The dewaterability of sludges that were conditioned with only TSIS (TSIS-sludge) or co-conditioned with TSIS and CPAM (flocculated TSIS-sludge) were assessed in terms of the net sludge solids yield (YN) and the time to filter (TTF). To measure the rate of filtration, a 500-ml conditioned sludge sample was poured into a Buchner funnel. The zeta potential, cake compressibility, porosity, floc size distribution and microstructure of the sludge samples were also measured to elucidate the mechanism for the observed changes in sludge dewaterability. 2.3. Analytical methods Sludge dewaterability was measured by the standard Buchner funnel test and was expressed in terms of the specific resistance to filtration (SRF) and the TTF. The SRF was obtained using the method described by Qi et al. [19]. The TTF is defined as the time required for the filtrate volume to increase up to half of the sludge volume; the TTF was obtained using the method described by Lo et al. [20]. The limitations of using the SRF to evaluate the performance of physical conditioners will be discussed in Section 3.1.1. The net sludge solids yield, YN (kg/m2 h), was used to evaluate the sludge filtration process with added conditioners [21]. YN physically corresponds to the rate of sludge solids filtered per unit area and unit time, which can be calculated as follows:

 YN ¼ K 

2Pw l  SRF  t

1=2 ð1Þ

where

K¼

Original sludge solids mass Original sludge solids mass þ Conditioner solids mass

ð2Þ

Table 1 Characteristics of the raw sludge sample.

2. Materials and methods 2.1. Test materials The sludge samples were collected from a tannery wastewater treatment plant in Dongguan City, China. The sludge samples con-

Parameters

Unit

Value

pH Moisture content Zeta potential Net sludge solids yield (YN) Time to filter (TTF)

– % mV kg/m2 h s

7.62–7.86 97.86–98.32 17.7–18.6 1.12–1.21 128–136

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In the expression above, K is a correction factor that is used to incorporate the effect of the added conditioners, t is the filtration time (s), l is the liquid viscosity (Pa s), and w is the mass of cake solids deposited per volume of filtrate and is assumed to be constant (kg/m3). The moisture content was measured in accordance with standard methods [22]. The pH was measured using a digital pH-meter (pHS-3C, LEICI, China). The flocs were broken up into small particles by shaking and then left to stand for 30 min; the supernatant was then sampled for zeta potential measurements by a Zetasizer Nano-Instrument (Nano-ZS90, England). An image analysis system was used to measure the size distribution of the flocs and the TSIS, using methods developed by He et al. [23] and Persson [24], respectively. The sludge compressibility was obtained using a method described by Zhao [5]. The sludge samples were tested under five pressure differentials ranging from 0.1 to 0.5 MPa. Mercury intrusion porosimetry (MIP) tests were performed on the filtrated cakes, by using a mercury porosimeter (mMK-AutoPore IV 9520, China) to determine the average cake porosity. The filtrated cakes were dried in an oven at 105 °C for 4 h prior to the porosimetry tests. Prior to the surface observation of the different sludge samples by SEM (S-3400N, Hitachi, Japan), the samples were rapidly frozen in liquid nitrogen and then vacuum dried. Each measurement was performed in triplicate and the average values were recorded. 3. Results and discussion 3.1. Dewatering performance 3.1.1. Dewatering characteristics of TSIS-sludge The effect of TSIS conditioning on the sludge dewaterability was investigated by measuring the TTF, SRF and YN for the TSIS-sludge. The experimental results are presented in Fig. 1. The YN and TTF values of the raw sludge were approximately 1.15 kg/m2 h (corresponding to a SRF of 49.5  1010 m/kg) and 132 s, respectively, showing poor dewaterability of the sludge. An increase in the TSIS dosage from 0% to 200% significantly improved the sludge dewaterability. For a TSIS dosage of 200%, the YN and TTF values rapidly reached 2.33 kg/m2 h and 42 s, respectively, corresponding to a 50.6% increase in the YN and a 69.8% decrease in the TTF relative to the untreated sludge. The sludge dewaterability, in terms of the YN and TTF values, did not improve with increasing the TSIS dosage beyond 200%. A significant correlation clearly existed between the YN and the TTF values. However, the dewatering performance based on the SRF measurements was different from that based on the TTF and YN measurements. The SRF decreased with increasing TSIS dosages, even for doses over 200%.

140

2.4

50 40

YN

80

TTF SRF

1.5

30

10

1.8

TTF (s)

100

SRF (×10 m/kg)

2

YN (kg/m h)

120 2.1

20

60

3

The additional of large quantities of physical conditioners produced a slurry with a higher solids content, consisting of the original sludge solids and the conditioner solids. With increasing physical conditioner doses, the SRF value reflected the sludge characteristics less and decreased towards the SRF value of the physical conditioner [1]. A low specific resistance of the resulting solids mixture and a higher recovery rate of total solids by filtration does not necessarily result in a higher recovery rate for the original sludge solids. Therefore, the SRF should only be used to compare conditioners or conditioner doses when the quantity of sludge solids remains relatively constant, independent of the conditioner dose [21]. To evaluate a sludge filtration process in which solids are added, the dewatering performance should be estimated by to minimize the error caused by the additional TSIS particles. The excess sludge contained a large quantity of fine particles [25]. Deformation of these particles and a gradual decrease of the porosity of filter cake during the filtration process resulted in a decrease in the liquid permeability of the filter cake. The TSIS particles may contain many channels or voids and the particle surfaces are irregular [26]. The fine sludge particles were incorporated into these channels or voids in the TSIS particles during mixing with the sludge. The reduction of the fine particles in the sludge improved dewatering. 3.1.2. Dewatering characteristics of flocculated TSIS-sludge Fig. 1 shows that using TSIS as a skeleton builder to condition sludge can enhance dewaterability. The effects of TSIS and CPAM co-conditioning on sludge dewaterability were estimated by measuring the YN and TTF (Fig. 2). When the sludge was conditioned only with CPAM, the YN values decreased with increasing CPAM dosage (Fig. 2b). The optimal CPAM dosage for dewatering the flocculated sludge was 20 kg CPAM/t DS, producing an increase in the YN from 1.15 kg/m2 to 7.49 kg/m2 h, which corresponded to an increase of 551.3% relative to the raw sludge. Similarly, the TTF values decreased from 132 s for the raw sludge to 12 s (Fig. 2a). Increasing the CPAM dosage beyond 20 kg/t DS negligibly improved the sludge. These results confirmed the well-established enhancement of sludge dewaterability by CPAM addition. When the sludge was pre-conditioned with low TSIS dosages, the sludge dewaterability still improved significantly with increasing CPAM dosages (Fig. 2). For a fixed CPAM dosage, the YN first increased and then decreased with increasing TSIS dosages. At CPAM dosages of 5 kg/t DS, 10 kg/t DS, and 15 kg/t DS and TSIS dosages of 150%, 100%, and 150%, the maximum YN values were 5.48 kg/m2 h, 10.30 kg/m2 h and 8.90 kg/m2 h, respectively (Fig. 2b). These results represented a 209.6%, 153.7% and 39.1% reduction in YN, respectively, relative to conditioning by CPAM dosage alone. Similar results were obtained for the TTF values (Fig. 2a). A set of experiments was designed to assess the effects of the adding sequence of CPAM and TSIS on sludge dewaterability. The results in Table 2 show that the order of addition significantly affected the TTF and YN. Adding the TSIS after the polymer resulted in relatively poor dewaterability, while adding the TSIS before the polymer produced excellent results. When CPAM was added first, this reduces the opportunity for TSIS to gain a better structure. Moreover, the fine TSIS particles, which cannot flocculated by CPAM, result in the deterioration of dewatering. Whereas when TSIS was added before the CPAM, the CPAM appears to promote flocculation in both the sludge and the TSIS.

10

3.2. Effect of TSIS on filtering rate

40 1.2

0 0

50

100 150 TSIS dosage (%)

200

250

Fig. 1. Effects of TSIS dosage on YN and TTF.

The process of SRF is a better option for filter press dewatering because of the similar filtration behavior [27], but large errors have been found when 100-ml sludge samples have been used. This is because most of the water passes through the filter medium before

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(a)

Without CPAM 10 kg/t DS 20 kg/t DS

TTF (s)

100

specific cake resistance was approximately 27 times lower than that of the flocculated sludge alone. Zall et al. [28] also found that oily sludge conditioned with hydrated lime and fly ash took 10 and 15 min, respectively, to achieve primary consolidation, whereas the oily sludge conditioned with only polymer failed to achieve primary consolidation even after 35 h.

5 kg/t DS 15 kg/t DS

3.3. Variations in pH and zeta potential in flocculated TSIS-sludge

100

150

200

250

TSIS dosage (%)

(b)

12

Without CPAM 5 kg/t DS 10 kg/t DS 15 kg/t DS 20 kg/t DS

2 YN( kg/m h)

10

8

6

4

2 50

100

150

200

250

TSIS dosage (%) Fig. 2. Effects of TSIS and CAPM co-conditioning on TTF and YN.

the cake formation stage, so that the permeability is not accurately measured. To avoid this error, a 500-ml sludge sample was used in the vacuum filter test in this study. Vacuum filtration tests were conducted on four sludge samples, including raw sludge, TSIS-sludge (150% TSIS), flocculated sludge (20 kg CPAM/t DS), and flocculated TSIS-sludge (150% TSIS with 10 kg CPAM/t DS), using the optimum conditioner dosage (Fig. 2). A vacuum pressure (0.6 MPa) was applied to each sludge sample until the sample was considered to be in static equilibrium: the results are shown in Fig. S1 (Supplementary material). The flocculated TSIS-sludge was clearly superior to the TSIS-sludge or the flocculated sludge for improving the rate of sludge dewatering, although the extent of water removal at the end of the filtration tests was similar for all three sludges (approximately 80%). The experimental data showed that the flocculated TSIS-sludge took 15 min to reach equilibrium, whereas the flocculated sludge took approximately 50 min and the raw sludge took more than 3 h (not shown here). Thapa et al. [2] similarly reported that for a sample containing 40% sludge and 60% lignite (on a dry basis), the

Table 2 Effects of the adding sequence of CPAM and TSIS on sludge dewaterability. Adding sequence

CPAM–TSIS

TSIS–CPAM

Dosage TTF (s) YN (kg/m2 h)

CPAM: 10 kg/t DS, TSIS: 150% DS 7 53 10.31 2.58

10

pH

50

9 8 0

-5

Zeta potential (mV)

10

The effects of pH on the surface charge of the raw sludge were investigated by measuring the zeta potential at various pH levels. Fig. S2 shows that the isoelectric point, which corresponds to a zero zeta potential, was obtained at a pH of around 2.60. The zeta potential decreased sharply when the pH rose from 2 to 6 and then decreased slightly for pH values above 6. Citeau et al. [29] has explained that the adsorption of hydroxide ions onto particles increase the negative particle surface charge under alkaline conditions. The zeta potential for the raw sludge (pH = 7.72) was 18.1 mV, corresponding to a net negative charge for the sludge particles. The excess sludge particles repelled each other via electrostatic interactions, thus forming a relatively stable system characterized by poor sludge settling and dewatering [30]. Fig. 3 illustrates the effects of the TSIS dosage on the zeta potential and the pH. For the TSIS-sludge, the magnitude of the zeta potential and the pH gradually increased with increasing TSIS doses. Many silicon and aluminum active sites with positive charges can appear on the sludge incineration slag surface, for example, aluminum silicate, ferric metasilicate and calcium silicate [31]. Therefore, the negative charge of the sludge can be neutralized by the positive charge from the aluminum silicate, ferric metasilicate and calcium silicate on the TSIS surface. Such a mechanism explains the destabilization and subsequent aggregation of the colloidal sludge particles. Fig. 3 confirms the well-established enhancement of the zeta potential by CPAM addition. This phenomenon can be attributed to the decrease in the electrostatic repulsion between the charged particles upon CPAM addition [7]. The zeta values increased from 18.1 mV for the raw sludge to 5.6 mV for the TSIS pre-conditioned sludge, at a dosage of 200% DS, and to 9.7 mV for the CPAM flocculated sludge, at a dosage of 20 kg/t DS. For the flocculated TSIS-sludge, the zeta potential further increased with increasing CPAM dosages. At a TSIS dosage of 150% and CPAM dosages of

-10

Without CPAM -15

10 kg CPAM/t DS 20 kg CPAM/t DS

-20 0

50

100

150

200

TSIS dose (%) Fig. 3. Effect of TSIS dosage on pH and zeta potential under various CPAM dosages.

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Flocculated sludge (10kg CPAM /t DS)

12

Size distribution (%)

10

TSIS

8

Size distribution (%)

14 8 6 4 2 0 0.0 0.2 0.4 0.6 0.8 1.0 1.2

6

Effective diameter (mm)

4 2

Flocculated TSIS-sludge (150 % TSIS+10 kg CPAM/t DS)

0 0

1

2

3

4

5

6

7

8

Effective diameter (mm)

5

while the flocculated TSIS-sludge was characterized by a dp 90 of 5.12 mm and a mean diameter of 3.48 mm. Fig. 3 shows that the negative charges were neutralized and the surface double electric layers were destroyed following TSIS addition, resulting in colloidal aggregation. Furthermore, the unique structural characteristics of the TSIS promoted the flocculation of destabilized particles, providing core frameworks for floc aggregation. Therefore, the floc sizes of the flocculated TSIS-sludge were larger than that of the flocculated sludge. The presence of colloidal and supra-colloidal (>1 lm) particles in sludge often deteriorates mechanical dewatering [1]. The TSIS also contained a large quantity of fine particles [32], as evidenced by the TSIS particle size distribution in Fig. 4. However, using both TSIS and CPAM as conditioners sharply reduced the number of the fine particles in the sludge. This result showed that the CPAM promoted the flocculation of both the sludge and TSIS particles, whereby the fine sludge particles fitting into channels or voids around the larger TSIS particles.

Fig. 4. Floc size distribution of the flocculated sludge and flocculated TSIS-sludge.

3.5. Microscopic structure 10 kg/t DS and 20 kg/t DS, the zeta potential values were 3.3 mV and 1.6 mV, respectively. Although the zeta potential approached zero with optimum conditioning, the zeta potential values did not increase linearly with increasing TSIS dosage. This result may be explained by the gradual increase in the pH, which greatly affects the zeta potential, with increasing TSIS doses.

3.4. Variations of floc size distribution in flocculated TSIS-sludge Fig. 4 compares the floc size distributions for the flocculated sludge (10 kg CPAM/t DS) and the flocculated TSIS-sludge (150% TSIS + 10 kg CPAM/t DS). The experimental data for the flocculated sludge showed that the 90th percentile (by volume) particle diameter (dp 90) was 4.12 mm and the mean diameter was 2.23 mm,

The microscopic structure of the freeze-dried samples was visualized to confirm the porosity of the sludge cake, as shown in Fig. 5. The SEM images of the TSIS samples (Fig. 5a) revealed an irregular structure with many channels or voids. The surface structures of the raw sludge (Fig. 5b) and flocculated sludge (Fig. 5c) were rather similar, indicating that both sludges formed a smooth surface with no channels or voids. This is because high cake compressibility facilitates the closure of voids in the raw sludge and flocculated sludge. Fig. 5d shows that the flocculated TSIS-sludge cake formed a discontinuous surface with many channels or voids, similar to the TSIS sample. The relatively incompressible TSIS particles can maintain channels or voids, thereby preventing the formation of an impermeable thin layer of sludge on top of the filter medium during dewatering by mechanical filtering [2].

Fig. 5. Microphotographs of sludge cake. (a) TSIS, (b) flocculated sludge (20 kg CPAM/t DS), (c) raw sludge, (d) flocculated TSIS-sludge (150% TSIS + 10 kg CPAM/t DS).

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6

Mercury intruded (mL/g)

SRFi/SRF0

4

0.20

Raw sludge TSIS-sludge(150% TSIS) flocculated sludge (20kg CPAM /t DS) flocculated TSIS-sludge (150%TSIS+10kgCPAM/t DS) S=0.96 S=0.86

S=0.71

2

S=0.40

raw sludge TSIS-sludge flocculated sludge (20kg CPAM /t DS)

0.15

flocculated TSIS-sludge (150%TSIS+10kgCPAM/t DS)

0.10

0.05

0.00

2

4

0.01

6

0.1

1

Pore diameter (mm)

Pt,i/Pt,o

Fig. 7. Effect of TSIS and CPAM on porosity of filtrated cake. Fig. 6. Log–log plot of SRFi/SRF0 as a function of pressure ratio at various conditioning. Pt,i, Pt,0 refer to the applied pressure and a reference pressure, respectively. SRFi and SRF0 are the corresponding values of the specific resistance. Parameter s can be obtained from a log–log plot of SRFi/SRF0 against the pressure ratio.

3.6. Filtrated cake compressibility The characteristics of sludge compressibility and porosity are critical for obtaining a higher solids content and to further sludge water removal [33]. Fig. 6 shows the effects of TSIS conditioning on the sludge compressibility (S). A small reduction (of approximately 0.1) was observed in the flocculated sludge compressibility (S = 0.86) relative to the compressibility of the raw sludge of 0.96. These results generally suggest that the raw sludge and flocculated sludge were characterized by a low degree of floc compaction. In addition, the dewaterability tests (Fig. S1) showed that the dewaterability of flocculated sludge was superior to that of the TSISsludge; however, Fig. 6 shows that the compressibility of the flocculated sludge was lower than that of the TSIS-sludge (S = 0.72). There are various reasons for the differences in the compressibilities. The TSIS particles were firm with irregular surfaces (Fig. 5a). These particles had a permeable and more rigid lattice structure that maintained its original shape even under high pressure. As a result, TSIS addition improved the cake porosity through the formation of channels or pores, decreasing the compressibility. In contrast, conditioning with only TSIS produced little improvement in the sludge dewaterability. TSIS is not a flocculant and could not flocculate the sludge particles, unlike CPAM. The flocculated TSIS-sludge compressibility was significantly lower than that of the flocculated sludge or the TSIS-sludge. The compressibility of the flocculated TSIS-sludge reached 0.43, corresponding to a reduction of 53.4% and 43.7% compared to the flocculated sludge and the TSIS-sludge compressibilities, respectively. These results indicate that the TSIS only improves sludge dewatering in combination with CPAM.

3.7. Filtrated cake porosity MIP tests were performed on the dried cakes; however, a significant amount of shrinkage can occur during the drying process. In addition, crack formation is also inevitable during sludge drying because of uneven thermal stresses [34]. The crack increases in thickness during the drying process, changing from having a flaky texture to a banded structure [35]; thus, the dry cake has a blocky structure with few cracks. The solid samples used for the mercury porosimetry tests were crushed into small pieces to fit into the

penetrometers. Thus, most of the cracks were removed during the crushing process. In the early stages of the mercury porosimetry tests, the channels or pores around the crushed pieces, as well as the cracks, were filled with mercury. These impacts above were ruled out during the porosity calculations. As a result, the formation of cracks had little effect on the mercury porosimetry tests. The surface morphology of different materials usually exhibits a high degree of self-similar behavior, corresponding to parts of an image resembling the whole image [35]. The true volume of the intruded mercury was obtained by correcting the data for sample compression, as described by Thapa et al. [2]. The MIP test results are shown in Fig. 7. The filtrated cake of the flocculated TSIS-sludge clearly had the highest porosity and the raw sludge had the lowest porosity. The flocculated sludge did not present any obvious advantages over the raw sludge for improving the porosity of the filtrated cake. The high compressibility of the flocculated sludge facilitated cake void closure, which impeded further dewatering [6]. The porosity of the filtrated cake increased significantly after TSIS conditioning, with or without CPAM, indicating that the pores or channels were created by the TSIS particles in the filtrated cake. In contrast, the flocculated TSIS-sludge had a higher porosity than the TSIS-sludge. 4. Conclusions The effects of TSIS and CPAM on tannery sludge dewatering were experimentally measured in this research. The dewatering rate of the flocculated TSIS-sludge was higher than that of the flocculated sludge or the TSIS-sludge. The zeta potential, floc size, compressibility and porosity were found to play a vital role in the observed changes in the sludge dewaterability. Conditioning the sludge with TSIS destroyed the surface double electric layers, resulting in aggregation of the sludge colloidal particles. Compressibility and porosity measurements and SEM images further demonstrated that the flocculated TSIS-sludge reduced the sludge compressibility by incorporating rigid structures into the sludge and creating passages for water removal by increasing the solids porosity, whereas the flocculated sludge by itself formed a highly compressible structure. Acknowledgements This research was supported by the Project of Special Enterprise Planning of Guangdong Province, at the Ministry of Education and Science and Technology (No. 2011B090400161). The authors

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