Stratification Structure of Sludge Flocs with Implications to Dewaterability

Sep 23, 2008 - Sludge dewatering has proven to be an effective method to reduce the volume of sludge. In this study, a novel stratification approach a...
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Environ. Sci. Technol. 2008, 42, 7944–7949

Stratification Structure of Sludge Flocs with Implications to Dewaterability GUANG-HUI YU, PIN-JING HE,* LI-MING SHAO, AND PEI-PEI HE State Key Laboratory of Pollution Control and Resources Reuse, Key Laboratory of Yangtze River Water Environment, College of Environmental Science and Engineering, Tongji University, Shanghai 200092, P.R. China

Received March 8, 2008. Revised manuscript received August 2, 2008. Accepted August 10, 2008.

Sludge dewatering has proven to be an effective method to reduce the volume of sludge. In this study, a novel stratification approach aimed at better understanding the factors influencing the sludge dewaterability (as determined by capillary suction time, CST) was developed. The sludge flocs from 14 different fullscale wastewater treatment plants (WWTPs), including sewage, leachate, industry, and special-source sludge, were stratified through centrifugation and ultrasound into five layers: (1) supernatant, (2) slime, (3) loosely bound extracellular polymeric substances (LB-EPS), (4) tightly bound EPS (TBEPS), and (5) pellet. The results showed that the distribution pattern of proteins (PN) in the sludge flocs differed from that of polysaccharides (PS). The normalized CST correlated with PN (R2 > 0.72, p < 0.01) and PN/PS (R2 > 0.51, p < 0.01) in the supernatant, slime, and LB-EPS, but not with PN and PN/ PS in the pellet and the sludge flocs as a whole or with PS in any of the fractions and or the sludge flocs as a whole. The results suggest that PN and PN/PS in the supernatant and slime layers, which are usually decanted due to their assumed lower content of organic matter, markedly impact sludge dewaterability.

Introduction Reducing the volume of waste sludge has been the focus for decades in wastewater treatment research (1). Sludge dewatering has proven to be an effective method to reduce the volume of sludge. Many characteristics are reported to influence the dewaterability of sludge, such as particle size, proteins (PN), polysaccharides (PS), and the PN/PS ratio in extracellular polymeric substances (EPS) or sludge flocs (2-4). Degradation of the EPS could release the EPS-bound water and increase the dewatering efficiency of sludge flocs (5). However, controversy remains concerning the extracellular organic characteristics that influence dewaterability, owing to the complex dynamics of the sludge matrix and the lack of well-defined methods to determine sludge structure. EPS and cells form bioaggregates, such as biofilms, sludge flocs, and “river snow” (lotic microbial aggregates) (6, 7). EPS in sludge flocs is composed of soluble EPS (i.e., slime) and bound EPS. The latter exhibits a dynamic double-layered structure and can be classified as loosely bound EPS (LBEPS) and tightly bound EPS (TB-EPS) based on the extraction * Corresponding author phone: +86-21-6598 6104; fax: +86-216598 6104; e-mail: [email protected]. 7944

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methodology (8-10). After the EPS is extracted, the cells in the residue form a pellet (11, 12). Hence, from their outer surfaces to the cores of their granules, sludge flocs possess a multilayered structure consisting of supernatant (bulk solution), slime, LB-EPS, TB-EPS, and pellet. To the best of our knowledge, the comparison of chemical compositions in these layers has not been studied with respect to the dewaterability of sludge flocs. Here we describe a well-defined stratification of sludge flocs that offers a novel approach to identifying the factors that influence sludge dewaterability, as determined by capillary suction time (CST). The CST was a rapid indication of the filterability of sludge, with results of good reproducibility. Meanwhile, it had good correlation with specific resistance filtration (SRF) (13). Centrifugation and ultrasound were used to stratify sludge flocs into five layers: (1) supernatant, (2) slime, (3) LB-EPS, (4) TB-EPS, and (5) pellet. The objectives of this study were to use this stratification to investigate the distribution patterns of PN and PS in the different layers of sludge flocs and to explore the factors that determine sludge dewaterability. Sludge flocs from 14 fullscale wastewater treatment plants (WWTPs), yielding sludge of sewage, leachate, industrial, and special-source (beer, cola, paper mill, and slaughterhouse) wastewaters, were obtained to identify the general stratification structure of sludge flocs. These allowed us to show that the structure of sludge flocs is not sludge-specific.

Materials and Methods Sludge Samples. Activated sludge samples were collected from the 14 full-scale WWTPs in Shanghai, China. These plants treated wastewater from sewage, leachate, industry, or special sources (Table 1). The collected samples were transported to a laboratory within 2 h after sampling. The samples were subsequently screened through a 1.2 mm sieve to remove grit and then stored at 4 °C and analyzed within 2 days. Stratification Structure and Protocol. Three spatial scales of sludge flocs were considered in this study (Figure 1a). The dispersed bacterium scale describes the supposed stratification structure for individual bacterium. The aggregate scale describes the spatial structure of the settled sludge flocs. The reactor scale describes the supernatant and settled sludge flocs in the reactor. The supposed stratification structure developed here corresponds to the reactor scale. Since the EPS matrix was shear sensitive (14), centrifugation and ultrasound could be applied to selectively remove the different EPS layers. Figure 1b illustrates the centrifugationand ultrasound-based method used to achieve the supposed stratification of the sludge flocs. In brief, screened raw sludge was allowed to settle for 1.5 h at 4 °C, after which the bulk solution, comprising the supernatant, was collected carefully by a siphon. The sediments were then centrifuged at 2000g for 15 min. The bulk solution was collected as the slime, i.e., the fraction that could be removed by soft centrifugation. The bottom sediments were collected and resuspended to their original volumes using a pH 7 buffer solution consisting of Na3PO4, NaH2PO4, NaCl, and KCl (6, 15). The molar ratio of these components was 2:4:9:1. The conductivities of the buffers were adjusted with distilled water to match those of the sludge sediment samples listed in Table 1. The suspensions were centrifuged at 5000g for 15 min, and the bulk solution and solid phase were collected separately. The organic matter in the bulk solution comprised the LB-EPS. The collected sediments were resuspended with the aforementioned buffer to the original volumes and then extracted 10.1021/es8016717 CCC: $40.75

 2008 American Chemical Society

Published on Web 09/23/2008

TABLE 1. Characteristics of the Sludge Sediment Sample characteristics of the sludge sediment sample symbol processesa

sludge sources

TSS(g/L)

conductivity (µS/cm) 331 338

29.9 ( 0.2 17.5 ( 0.1 31300 ( 1900 13.3 ( 0.3 8.0 ( 0.1 21300 ( 1800 17.6 ( 1.7 7.6 ( 0.9 17200 ( 1300 10.9 ( 0.1 4.8 ( 0.1 8100 ( 1700 5.6 3.9 12000 ( 1400

531 ( 46 447 ( 15 545 ( 26 782 ( 11 617 ( 38

8.3 8.7 7.7 7.8 8.1

1137 699 1034 1614 558

SBR SBR SBR

12.0 ( 0.6 11.8 ( 0.8 13.9 ( 0.4

6.9 ( 0.5 22000 ( 1000 6.7 ( 0.4 3800 ( 686 4.8 ( 0.2 3600 ( 42

226 ( 7 10 ( 7 3(1

7.0 6.8 6.8

587 391 340

UASB AO SBR AO

38.2 ( 0.7 6.9 ( 0.3 11.7 ( 0.3 8.5 ( 0.2

8.3 ( 0.2 13800 ( 400 4.7 ( 0.1 11200 ( 300 7.6 ( 0.1 31000 ( 2000 3.4 ( 0.1 6100 ( 900

124 ( 21 115 ( 45 0 59 ( 4

7.5 7.5 7.4 7.8

285 1223 443 860

A2O SBR

7.1 12.6 ( 0.1

leachate WWTP

L1 L2 L3 L4 L5

SBR A 2O AO MBR SBR

industry WWTP

I1 I2 I3 SS1 SS2 SS3 SS4

a

6.7 9100 ( 378 8.8 ( 0.2 16000 ( 1000

SCOD(mg/L) pH 6.7 6.6

S1 S2

beer WWTP cola WWTP Paper mill WWTP slaughterhouse WWTP

COD(mg/L)

94 ( 5 15 ( 3

sewage WWTPb

special sources

VSS(g/L)

A2O, anaerobic-anoxic-oxic process; SBR, sequencing batch reactor; MBR, membrane bioreactor. wastewater treatment plant.

b

WWTP,

FIGURE 1. Supposed stratification structure and extraction protocol used in analyzing the sludge flocs. (a) supposed stratification structure of sludge flocs at different scales; (b) the centrifugation- and ultrasound-based extraction protocol. Note: the actual layers did not exist in sludge flocs. Since the different layers possessed the different shear sensitivities, they were classified by the method used to extract them in this figure. by subjecting the suspension to ultrasound at 20 kHz and 480 W for 10 min. The extracted solutions were centrifuged at 20 000g for 20 min. Organic matter in the bulk solution comprised the TB-EPS; the residues (solid phase) were again resuspended with the aforementioned buffer to their original volumes. This fraction comprised the pellet. Polytetrafluoroethylene (PTFE) membranes (Mosu Scientific Equipment Co., Shanghai, China) with a pore size of 0.45 µm were used to remove the particulates present in the supernatant, slime, LB-EPS, and TB-EPS solutions. The reproducibility test and results of the stratification method were given in the Supporting Information (SI) as Tables S1-S3. CST Determination. The dewaterability of the sludge flocs was measured with a CST instrument (model 319, Triton,

UK) equipped with an 18 mm diameter funnel and Whatman no. 17 chromatography-grade paper. The CST values were normalized by dividing them by the initial TSS concentration and then expressed in units of seconds per liter per gram TSS (16). Sludge Characterization Assay. All chemical analyses were carried out in duplicate using chemicals of analytical grade. PN was determined by the modified Lowry method (15), using casein (Shanghai Sangon Biotechnology Co., Ltd., China) as the standard. PS was measured by the anthrone method (17), with glucose as the standard. The chemical oxygen demand (COD) of the sludge solution filtered through a 0.45 µm PTFE membrane was referred to as the soluble COD (SCOD). COD and SCOD were analyzed using a HACH DR/2000 Spectrometer. Conductivity was determined with VOL. 42, NO. 21, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 2. Proteins (PN) and polysaccharides (PS) of the different sludge layers in sludge flocs obtained from 14 WWTPs. (a) PN content, (b) PN percentage, (c) PS content, (d) PS percentage. Median, 1st, 25th, 75th, and 99th percentiles are plotted as horizontal solid lines of the boxes. The square indicates the mean content value. Minimum and maximum values are shown as a star. a conductivity meter (DDSJ-308A, Leici Co., Ltd., Shanghai, China). Other sludge parameters, including total suspended solids (TSS) and volatile suspended solids (VSS), were analyzed following standard methods (18). Statistical Analysis. Statistical analysis was carried out using the software SPSS version 11.0 for Windows (SPSS, Chicago, IL). Pearson’s correlation coefficient (R) was used to evaluate the linear correlation between two parameters. Pearson’s coefficient is always between -1 and +1, where -1 denotes a perfect negative correlation, +1 a perfect positive correlation, and 0 the absence of a relationship. The correlations were considered statistically significant at a 95% confidence interval (p < 0.05).

Results and Discussion Distributions of PN and PS in the Sludge Flocs According to the Stratification Structure. Figure 2 presents the PN and PS distributions in the sludge layers and sludge flocs from the fourteen WWTPs (the actual data is listed in the SI as Table S4). PN and PS concentrations in sludge flocs ranged from 66.6 to 902.1 mg casein/(g-VSS) and from 11.8 to 42.6 mg glucose/(g-VSS), respectively. The results showed that PN was the predominant component in the sludge flocs, with 71.4-99.5% of the PN distributed in the pellet and TBEPS layers, and only 0.5-28.6% in the LB-EPS, slime, and supernatant layers. Most of the PS (45.4-70.6%) was similarly present in the pellet and TB-EPS layers. However, in the LB-EPS, slime, and supernatant layers, the PS fraction was larger than the PN fraction (29.4-54.6% vs 0.5-28.6%). The distribution of PN and PS, therefore, differed within the sludge flocs. Specifically, PN was mainly distributed in the pellet and TB-EPS layers, with minor amounts in the LB-EPS, slime, and supernatant layers. Although PS was also mainly present in the pellet and TB-EPS layers, the percentage of PS in the LB-EPS, slime, and supernatant layers was higher than that of PN. More importantly, the distribution patterns were not sludge-specific since the same pattern was observed in each of the broad range of sludge types (the actual data is listed in the SI as Table S4). Normalized CST of the Sludge Flocs. The normalized CST provides a simple, rapid, and inexpensive means to measure the dewaterability potential of sludge flocs (16, 19). 7946

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FIGURE 3. Variations in normalized capillary suction time (CST) from sludge flocs of 14 wastewater treatment plants (WWTPs). (a) sewage sludge. (b) leachate sludge, (c) industry sludge, (d) special source sludge. The variations in normalized CST of the sludge flocs from fourteen WWTPs are shown in Figure 3. The normalized CST values of the sludge flocs from the leachate WWTP (except L1) ranged from 2.08 to 10.7 s L/g-TSS and were therefore higher than those (0.33-1.5 s L/g-TSS) of the sludge flocs from the other WWTPs. This finding suggested that the dewaterability of sludge flocs from the leachate WWTPs was much poorer than that of sludge flocs from the other types of WWTPs. The normalized CST values of the other sludge flocs apart from the leachate WWTPs were similar to those reported by Scholz (20). Pearson Correlation Analyses between PN, PS, PN/PS, and Normalized CST. Figures 4-6 depict the Pearson correlations between the PN, PS, and PN/PS of the different layers of the sludge flocs and the normalized CST. The normalized CST correlated with PN (R2 > 0.72, p < 0.01) and with PN/PS (R2 > 0.51, p < 0.01) in the supernatant, slime, and LB-EPS layers. There was no correlation with PN (R2 < 0.26, p > 0.06), or PN/PS (R2 < 0.11, p > 0.24) in the TB-EPS, pellet, or the sludge flocs as a whole, or with PS (R2 < 0.12,

FIGURE 4. Pearson correlation between normalized CST and PN from different sludge layers of 14 WWTPs. (a) supernatant, (b) slime, (c) LB-EPS, (d) TB-EPS, (e) pellet, (f) whole sludge flocs.

FIGURE 5. Pearson correlation between normalized CST and PS from different sludge layers of 14 WWTPs. (a) supernatant, (b) slime, (c) LB-EPS, (d) TB-EPS, (e) pellet, (f) whole sludge flocs. p > 0.22) in any of the fractions or the sludge flocs as a whole. Thus, sludge dewaterability was affected by PN and PN/PS in the supernatant, slime, and LB-EPS layers. Several studies have been aimed at determining the effects of LB-EPS and TB-EPS on the filterability or dewaterability of sludge flocs. Rosenberger and Kraume (4) examined the filterability of sludge in a membrane reactor and found that suspended EPS (corresponding to LB-EPS in this study) rather than extractable EPS (corresponding to TB-EPS in this study) affected the filterability of activated sludge. Other studies also noted that the LB-EPS accounted for most of the filtration resistances of whole sludge (21). Similar tests were conducted by Li and Yang (10), who reported that LB-EPS played a greater role than TB-EPS in dewatering (as determined by specific resistance to filtration, SRF). Few studies have examined the effects of the supernatant and slime fractions on the dewaterability of sludge flocs. In previous investigations, these fractions were usually decanted

since they were assumed to contain less organic matter (6, 15). Although Novak et al. (22) also found that the dewatering rate was correlated directly with the “supernatant” fraction, the supernatant in their works was achieved by 10 000g centrifugation for 15 min and corresponded to the summation of supernatant, slime, LB-EPS, and parts of TB-EPS in this study. Therefore, the results reported here demonstrate that the supernatant and slime fractions significantly impact the dewaterability of sludge flocs. Increased amounts of PN or PN/PS in the outer layers of sludge flocs, i.e., the slime and LB-EPS fractions, are likely to inhibit the attachment of bacteria, weaken the sludge matrix, and thus enhance the release of extracellular organic matter into the supernatant (23, 24). In turn, the released organic matter impedes the dewaterability of sludge flocs. It should be pointed out that an increase in PN or PN/PS in the whole sludge flocs does not lead to the deterioration of dewaterability (Figures 4 and 6). Moreover, the presence of VOL. 42, NO. 21, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 6. Pearson correlation between normalized CST and PN/PS from different sludge layers of 14 WWTPs. (a) supernatant, (b) slime, (c) LB-EPS, (d) TB-EPS, (e) pellet, (f) whole sludge flocs. PS in the different layers of sludge flocs did not significantly correlate with the dewaterability of sludge flocs. This result may in part be due to the larger molecular size of PN compared to PS (25). Novak et al. (22) showed that the influence of PN on sludge dewaterability was significant, whereas PS appeared to be of little importance. Implications for Dewatering. Stratification of the sludge flocs was determined based on a broad range of sludge types, which eliminated the possibility that our results were sludge-specific. Most previous investigations on EPS were actually the total EPS or TB-EPS (10). As a result, the influence of EPS on sludge dewaterability was still under debate (3, 8, 10). An analysis of the stratification in this study showed that sludge dewaterability correlated with both PN and PN/PS in the supernatant, slime, and LB-EPS layers, but not with PN and PN/PS in the TB-EPS, pellet, and the sludge flocs as a whole, or with PS in any of the fractions and the sludge flocs as a whole. Data in SI Table S5 also confirmed no statistically significant correlation (R2 < 0.11, p > 0.25) between normalized CST and any of total organic indices (i.e., TSS, VSS, COD) in sludge flocs. Conversely, strong correlation (R2 ) 0.31, p < 0.05) was noted between normalized CST and SCOD. Although Li and Yang (10) and Novak et al. (22) also found that sludge dewaterability was correlated with LB-EPS rather than TBEPS, or with the called “supernatant” fraction (i.e., the summation of supernatant, slime, LB-EPS and parts of TB-EPS in this study), the results reported here demonstrate that the supernatant and slime fractions have more important influence on sludge dewaterability than the LBEPS fraction. Therefore, sludge flocs should have good dewaterability if the organic material (mainly PN) embedded in the sludge matrix does not enter the outer layers, i.e., the supernatant, slime, and LB-EPS. In other words, sludge dewaterability is determined by the structure of sludge flocs. Many factors were reported to influence the sludge structure. Adav et al. (26) suggested that in aerobic granulation process, the strong shear force provided by aeration could “compact” aggregates into granules. Therefore, the shear force should be manipulated in order to obtain a compact structure of sludge flocs. Li and Yang (10) and Eriksson et al. (14) indicated that LB-EPS or weakly bound EPS was increasing with decreasing sludge age. Thus, sludge flocs 7948

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with long sludge age would have compact structure. In addition, sludge load also affected the sludge structure, i.e., low sludge load would generate the compact structure of sludge flocs, and vice versa (14). Hence, it was beneficial to manipulate the low sludge load in order to achieve the compact structure of sludge flocs. In conclusion, the structure of sludge flocs is capable of being manipulated by adjustment of process parameters. Moreover, the stratification of sludge flocs in the present study showed that PN was the strongest determinant of dewaterability. On the other hand, PN in sludge flocs was shown to have an affinity for iron (27, 28). Thus, it is likely that sludge dewaterability can be improved to a greater extent by adding iron than by the addition of other cations to sludge flocs. In summary, the supposed stratification of sludge flocs described here provides new insights into dewaterability. Increased knowledge about the distribution of PN and PS in the different layers of sludge flocs contributes to our understanding of sludge dewaterability and provides novel information for engineering applications as well as scientific research.

Acknowledgments We thank the National Hi-Tech Research and Development Program of China (2006AA06Z384).

Supporting Information Available Reproducibility of the supposed stratification method; Actual data about proteins and polysaccharides of different sludge layers in sludge flocs obtained from 14 WWTPs; Pearson correlation between normalized CST and characteristics of sludge flocs from 14 WWTPs. This material is available free of charge via the Internet at http://pubs.acs.org.

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