Guang-Hui Yu

Characteristics of extracellular polymeric substances (EPS) fractions from excess sludges and their effects on bioflocculability

Extracellular polymeric substances (EPS) of biological origin are ubiquitous in excess sludges and can be applied as an underlying bioflocculant, owing to their high content of macromolecules and cations. However, low flocculating activity limits the feasibility of their practical applications. This study provides a novel EPS fractionation approach to improve their flocculability by extracting an active EPS fraction and removing the others with low flocculability. The results showed that for two excess sludges (called sludge A and sludge B), the tightly bound EPS (TB-EPS) fraction possessed a high flocculating rate to kaolin suspension compared with the other EPS fractions [i.e., supernatant, slime, and loosely bound EPS (LB-EPS) fraction] (>54.1+/-1.4% vs <7.8+/-1.6%). High bioflocculability of TB-EPS fraction could be attributable to high contents of macromolecules (330-1200 kDa) and trivalent cations (Fe(3+) and Al(3+)). Further investigation reveals that the TB-EPS fraction caused aggregation of particles by bridging and sweep flocculation.

Novel insights into sludge dewaterability by fluorescence excitation-emission matrix combined with parallel factor analysis.

Sludge dewatering is of major interest in sludge volume reduction and handling properties improvement. Here we report an approach of fluorescence excitation-emission matrix (EEM) combined with parallel factor (PARAFAC) analysis to elucidate the factors that influence sludge dewaterability. Sludge flocs from 11 full-scale wastewater treatment plants were collected to stratify into different extracellular polymeric substances (EPS) fractions and then to characterize their fluorescence EEMs. Both the normalized capillary suction time (CST) and specific resistance to filtration (SRF) were applied to determine sludge dewaterability. The results showed that fluorescence EEMs of tightly bound fractions were not affected by the wastewater sources. In contrast, fluorescence EEMs of loosely bound fractions were affected by the wastewater sources. All the fluorescence EEMs could be successfully decomposed into a six-component model by PARAFAC analysis. Both normalized CST and SRF were significantly correlated with component 1 [excitation/emission (Ex/Em)=(220, 275)/350] in the supernatant fraction, with components 5 [Ex/Em=(230, 280)/430] and 6 [Ex/Em=(250, 360)/460] in the slime and LB-EPS fraction. These results reveal that except for proteins-like substances (component 1), sludge dewaterability is also affected by humic acid-like and fulvic acid-like substances (components 5 and 6) in the slime and LB-EPS fractions. Furthermore, this paper presents a promising and facile approach (i.e., EEM-PARAFAC) for investigating sludge dewaterability.

Toward understanding the mechanism of improving the production of volatile fatty acids from activated sludge at pH 10.0

A well-defined fractionation approach for sludge flocs was applied to a better understanding of the underlying mechanism of improving the production of volatile fatty acids (VFA) in the hydrolysis and acidification processes at pH 10.0. Specifically, sludge flocs were fractionated through centrifugation and ultrasound into four fractions: (1) slime, (2) loosely bound extracellular polymeric substances (LB-EPS), (3) tightly bound EPS (TB-EPS) and (4) pellet. Result showed that during 20 days of fermentation, the total VFA production at pH 10.0 was higher, from 2 to 34 times, than that at pH 5.5. At pH 10.0, however, the enzyme activities (i.e. protease, alpha-amylase, alkaline phosphatase and acid phosphatase) in all fractions of sludge flocs were all lower than pH 5.5, which strongly suggests that the biotic effect was not the leading cause of the VFA improvement. Further investigation suggests that pH 10.0 could significantly improve the VFA production through the break of sludge matrix which is usually hydrolyzed by the extracellular enzymes embedded in itself, increase the effective contact between extracellular organic matters and enzymes, and create a favorable environment for microbes to accumulate VFA. Hydrolysis and acidification at pH 10.0 can be considered as part of an appropriate solution for tertiary treatment and contribute to the headway toward the goal of sustainable water treatment technologies.

Enzyme activities in activated sludge flocs

This study quantified the activities of enzymes in extracellular polymeric substances (EPS) and in pellets. Seven commonly adopted extraction schemes were utilized to extract from aerobic flocs the contained EPS, which were further categorized into loosely bound (LB) and tightly bound (TB) fractions. Ultrasonication effectively extracted the EPS from sludge flocs. Enzyme assay tests showed that the protease activity was localized mainly on the pellets, α-amylase and α-glucosidase activities were largely bound with LB-EPS, and few protease, α-amylase, or α-glucosidase activities were associated with the TB-EPS fraction. There exists no correlation between the biochemical compositions of EPS and the distribution of enzyme activities in the sludge matrix. The 44–65% of α-amylase and 59–100% of α-glucosidase activities noted with the LB-EPS indicate heterogeneous hydrolysis patterns in the sludge flocs with proteins and carbohydrates.

Effects of ultrasonic pretreatment on sludge dewaterability and extracellular polymeric substances distribution in mesophilic anaerobic digestion

Effect of ultrasonic pretreatment on sludge dewaterability was determined and the fate of extracellular polymeric substances (EPS) matrix in mesophilic anaerobic digestion after ultrasonic pretreatment was studied. Characteristics of proteins (PN), polysaccharides (PS), excitation-emission matrix (EEM) fluorescence spectroscopy and molecular weight (MW) distribution of dissolved organic matters (DOM) in different EPS fractions were evaluated. The results showed that after ultrasonic pretreatment, the normalized capillary suction time (CST) decreased from 44.4 to 11.1 (sec x L)/g total suspended solids (TSS) during anaerobic digestion, indicating that sludge dewaterability was greatly improved. The normalized CST was significantly correlated with PN concentration (R2 = 0.92, p < 0.01) and the PN/PS ratio (R2 = 0.84, p < 0.01) in the loosely bound EPS (LB-EPS) fraction. Meanwhile, the average MW of DOM in the LB-EPS and tightly bound EPS (TB-EPS) fractions also had a good correlation with the normalized CST (R2 > 0.66, p < 0.01). According to EEM fluorescence spectroscopy, tryptophan-like substances intensities in the slime, LB-EPS and TB-EPS fractions were correlated with the normalized CST. The organic matters in the EPS matrix played an important role in influencing sludge dewaterability.

Effect of ultrasonic pretreatment on anaerobic digestion and its sludge dewaterability

To investigate the effect of ultrasonic pretreatment on anaerobic digestion and sludge dewaterability and further to probe into the influencing factors on sludge dewaterability, sludge flocs were stratified into four fractions: (1) slime; (2) loosely bound extracellular polymeric substances (LB-EPS); (3) tightly bound EPS (TB-EPS); and (4) EPS-free pellets. The results showed that ultrasonic pretreatment increased the anaerobic digestion efficiency by 7%-8%. Anaerobic digestion without ultrasonic pretreatment deteriorated the sludge dewaterability, with the capillary suction time (CST) increased from 1.42 to 47.3 (sec x L)/g-TSS. The application of ultrasonic pretreatment firstly deteriorated the sludge dewaterability (normalized CST increased to 44.4 (sec x L)/g-TSS), while subsequent anaerobic digestion offset this effect and ultimately decreased the normalized CST to 23.2 (sec x L)/g-TSS. The dewaterability of unsonicated sludge correlated with protein (p = 0.003) and polysaccharide (p = 0.004) concentrations in the slime fraction, while that of sonicated sludge correlated with protein concentrations in the slime and LB-EPS fractions (p < 0.05). Fluorescent excitation-emission matrix analysis showed that the fluorescence matters in the LB-EPS fraction significantly correlated with sludge dewaterability during anarobic digestion.

Enhanced aerobic granulation with extracellular polymeric substances (EPS)-free pellets.

Extracellular polymeric substances (EPSs) were secreted by cells after they agglomerated into a compact aggregate. This study shows that the EPS initially embedded in seed sludge before granulation may sterically slow subsequent microbe-microbe contact, thereby delaying aerobic granulation. Three identical bioreactors were used in this study using glucose as the sole carbon and energy source. Reactor 1 (R1) was seeded with EPS-free pellets and operated in sequencing batch reactor (SBR) mode. Reactor 2 (R2) was seeded with the original sludge flocs and operated in SBR mode. Reactor 3 (R3) was seeded with EPS-free pellets and operated in continuously stirred tank reactor (CSTR) mode. Granulation occurred in R1 earlier than in R2; the granules that formed in R1 were larger and more compact than those in R2 at the same cultivation time. The few mature granules in R3 suggest that aerobic granulation can occur in a CSTR when a reactor is seeded with EPS-free pellets.

Enhanced storage stability of aerobic granules seeded with pellets

The responses of two different types of aerobic granules to storage, granule A seeded with activated sludge flocs and granule B seeded with pellets (cells), were investigated in this study. After 3-week storage, the surface of granule B remained compact and smooth while obvious crevices were observed on that of granule A. Compared with granule B, granule A had more decrease in biomass concentration, settleability, hydrophobicity, and extracellular polymeric substances (EPS) concentration after the storage. Results indicated that the stability loss of aerobic granules could be related to protein concentration decrease in the TB-EPS fraction and to protein framework disintegration in whole granule. Compared with aerobic granules seeded with activated sludge flocs, those seeded with pellets were more resistant against storage, and thus would have greater potential in practical applications.

Fouling Layer with Fractionated Extracellular Polymeric Substances of Activated Sludge

The soluble microbial products (SMP) and extracellular polymeric substances (EPS) significantly affect fouling in membrane filtration. This work filtered SMP and fractionated EPS of activated sludge using a 0.45 mm filter membrane. The fluorescently labeled stains and confocol laser scanning microscopy (CLSM) images in situ visualized the distributions of proteins, α-polysaccharides, β-polysaccharides, lipids, total cells, and dead cells of different EPS fractions in the fouling layers. The role of individual SMP and fractionated EPS fractions on membrane fouling was discussed.