Tightly Bound EPS Research Articles

Piggery wastewater treatment by aerobic granular sludge: Granulation process and antibiotics and antibiotic-resistant bacteria removal and transport

The aim of this work was to study the responses of aerobic granulation process to antibiotics and investigate the antibiotics and antibiotic-resistant bacteria (ARB) removal and transport. Results showed that aerobic granular sludge (AGS) was dominant in the bioreactor at day 45, and the relatively high protein content from tightly bound extracellular polymeric substances (TB-EPS) facilitated aerobic granulation and maintained biomass stabilization. The protein contents in EPS and TB-EPS were positively correlated with relative hydrophobicity, thereby improving the adsorption capacity among hydrophobic particles. The chemical oxygen demand (COD), NH3-N, and total N removal efficiencies were 98.0%, 97.0%, and 92.4%, respectively. Five antibiotics, including kanamycin, tetracycline, ciprofloxacin, ampicillin, and erythromycin, were examined in piggery wastewater, with concentrations up to the concentration range of 29.4–44.1 µg/l, and the total antibiotics removal rate reached up to 88.4% ± 4.5%. A total of 5.2% of the total antibiotics were discharged from bioreactor, and 62.5% of the total antibiotics were degraded, and 32.3% of total antibiotics were adsorbed by aerobic granules. The presence of antibiotics rarely exhibited an influence on AGS formation, and the relatively high microbial activity of aerobic granules was beneficial to antibiotics removal. The ARB removal rate increased up to 89.4% ± 3.3%, but a large amount of ARB was enriched in aerobic granules.

Reactivation and pilot-scale application of long-term storage denitrification biofilm based on flow cytometry

The work provides a method on the basis of flow cytometry to evaluate the performance of denitrification biofilm during the preservation, reactivation and pilot-scale operation process. The viable cell ratio of denitrification biofilm significantly reduced and further led to the decrease of denitrification capacity after long-term preservation for 5 months. Protein component in tightly bound extracellular polymeric substances (TB-EPS) could serve to enhance microbial adhesion and promote denitrification biofilm formation. With the significant correlation of viable cell ratio and microbial characteristics, 4 °C was more appropriate for preserving denitrification biofilm and conducive to maintain the relatively high denitrification capacity. A maximum denitrification rate of 5.80 gNO3--N/m2·d was obtained in pilot-scale anoxic-oxic (AO) process and Dechloromonas became greater prevalence in denitrification suspended carriers. Furthermore, the enrichment of Pseudomonas, Parcubacteria, Acidovorax, Aquabacterium and Unclassified_Flavobacteriaceae enhanced biofilm formation and nutrient conservation. The significantly positive correlation between viable cell ratio and the ratio of nitrate reduction to COD consumption was discovered, and the indices of Chao, ACE, Shannon and Simpson of denitrification biofilm were positively correlated with viable cell ratio, meaning that flow cytometry analysis was reasonable and suitable to evaluate the performances of denitrification biofilm.

Application of biochar in a CIC reactor to relieve ammonia nitrogen stress and promote microbial community during food waste treatment

Ammonia nitrogen inhibition is a well-known issue in the anaerobic digestion of food waste. This study aims to investigate the influence of biochar on the performance and the microbial community structure of a controlling internal circulation (CIC) reactor for food waste treatment in presence of 1500 mg/L of ammonia nitrogen. When ammonia nitrogen concentration increased to 1500 mg/L in the influent, chemical oxygen demand (COD) removal efficiency was reduced to 76%, the fluorescence peaks of aromatic proteins disappeared, and the intensity of coenzyme F420 was reduced in the three-dimensional excitation emission matrix (3D-EEM) spectra of soluble microbial products (SMP) and tightly-bound extracellular polymeric substances (TB-EPS). The band intensity at 1635 cm−1, attributed to the C=O stretching vibrations, reduced, and the broad band values of 3400 cm−1 implied a sharp peak of the amino group in the fourier transform infrared (FTIR) spectra of TB-EPS. In the CIC reactor, COD removal efficiency reached to about 90% with biochar. High-throughput sequencing revealed that the abundance of Clostridiales, Lactobacillales and Methanoculleus increased, while the abundance of Syntrophomonas decreased at high ammonia nitrogen concentrations. In presence of biochar, the abundance of Methanoregulaceae, Bacteroidales, Anaerolineales, and Syntrophobacterales increased. It was concluded that biochar addition could alleviate the inhibitory effect of high ammonia nitrogen concentration.

Synergistic effects of wheat straw powder and persulfate/Fe(II) on enhancing sludge dewaterability

The effects of wheat straw powder (WSP) used as physical conditioner on sludge dewatering was investigated under sodium persulfate (SPS)/Fe(II) oxidation. Sludge dewatering performance in terms of capillary suction time (CST), specific resistance to filtration (SRF) and moisture content (MC) was enhanced with increasing WSP and SPS dosages. The results showed presence of synergistic effect in WSP and SPS conditioning system, with sludge CST and SRF reduced by 43.9% and 65.6%, respectively, after dosing 0.75 g/g DS (dry solid) WSP, 120 mg/g DS SPS and 33 mg/g DS Fe(II), indicating that sludge dewatering became more easily. Correspondingly, bound water was released and decreased from 5.75 g/g DS to 1.5 g/g DS and deep dewatered sludge MC reached to 58.2% under 2 MPa pressure. Mechanically, tightly bound extracellular polymeric substances (TB-EPS) with larger molecular weights were oxidized and degraded into loosely bound-EPS (LB-EPS) and soluble organic matter with smaller molecular weights by SPS/Fe(II). Additionally, the organic matters released from or still in WSP was also oxidized resulting in more channels and less fine particles.

Ferroferric Oxide Significantly Affected Production of Soluble Microbial Products and Extracellular Polymeric Substances in Anaerobic Methanogenesis Reactors.

Conductive materials facilitate direct interspecies electron transfer between acidogens and methanogens during methane (CH4) production. Soluble microbial products (SMP) and extracellular polymeric substances (EPS) produced by microorganisms might act as the electron shuttle between microorganisms and conductive materials. In this study, effects of conductive ferroferric oxide (Fe3O4) on anaerobic treatment process and the production of SMP and EPS were investigated. The maximum CH4 production rate was enhanced by 23.3% with the dosage of Fe3O4. The concentrations of proteins, polysaccharides, and humic substances in tightly bound EPS (T-EPS) were promoted, suggesting that extracellular metabolisms were induced by conductive materials. Distribution of potential electron shuttles such as quinone-like substances, flavins, aromatic amino acids, and dipeptides in SMP and EPS phases were comprehensively investigated and these electron shuttles were significantly affected by Fe3O4. Dipeptides consisting of phenylalanine were widely detected in T-EPS of the Fe3O4 reactor, indicating a potential different extracellular electron exchange pattern with the addition of conductive materials.

Mechanisms and characteristics of biofilm formation via novel DEAMOX system based on sequencing biofilm batch reactor

A denitrifying ammonium oxidation (DEAMOX) process has been regarded as an innovative process to simultaneously treat ammonia and nitrate containing wastewaters, whereas very limited research has focused on its application in biofilm system. In this research, a novel DEAMOX process was established with fixed sponge carriers in a sequencing biofilm batch reactor (SBBR). To investigate biofilm formation process and characteristics can encourage further research on DEAMOX system optimization, deteriorated performance recovery strategies and application with actual wastewater. Total nitrogen removal efficiency was maintained at 93.0 % after 240 days of operation. With biofilm growth, the protein-like extracellular polymeric substances (EPS) and tightly-bound EPS (TB-EPS) of biofilms increased from 65.6 to 46.1, to 179.6 and 142.0mg gVSS-1, respectively, revealing that protein-like substances and TB-EPS promote biofilm formation. The mechanism of biofilm formation was discussed by analyzing the morphological development and functional bacterial activities of biofilms. Furthermore, high anammox activity was obtained in biofilms with specific NH4+N removal rates over 4.29 mgN gVSS-1h-1, which were significantly higher than in suspended sludge (2.56 mgN gVSS-1h-1). Quantitative polymerase chain reaction results showed that the abundance of anammox bacteria in biofilms increased from 1.87 % to 11.48 % with biofilm growth. These results imply that mature biofilms formed on carriers and the anammox bacteria were sufficient enriched in DEAMOX-SBBR system.

Effect of Fe3O4 nanoparticles on composition and spectroscopic characteristics of extracellular polymeric substances from activated sludge

Abstract The composition and spectroscopic characteristics of extracellular polymeric substances (EPS) from activated sludge were analyzed in a sequencing batch reactor under Fe3O4 nanoparticles (Fe3O4 NPs) stress. Fe3O4 NPs promoted the activated sludge to secrete more total EPS, loosely bound EPS (LB-EPS) and tightly bound EPS (TB-EPS) under long-term exposure. The protein and polysaccharide contents in the LB-EPS and TB-EPS displayed an increasing trend with the increase of Fe3O4 NPs concentration from 0 to 60 mg/L. The protein-like substances belonged to dominant fluorescence components in the EPS, whereas their peak location and intensity in the excitation-emission matrix fluorescence spectra had some evident changes under different Fe3O4 NPs concentrations. The groups of −OH and –NH2 branches of hydroxyl and amine groups in the LB-EPS and TB-EPs were sensitive to Fe3O4 NPs. The O/C ratio in the LB-EPS and TB-EPS was more than the N/C ratio at each Fe3O4 NPs concentration, suggesting that the mass fraction of element was in the order of C > O > N.

Granulation process in an expanded granular sludge blanket (EGSB) reactor for domestic sewage treatment: Impact of extracellular polymeric substances compositions and evolution of microbial population

In this study, an expanded granular sludge blanket (EGSB) reactor was used for the treatment of low-strength domestic sewage and the sludge granulation process was systematically investigated. At an optimized hydraulic retention time (HRT) of 5 h, up-flow velocity (Vup) of 1.9 m/h, and organic loading rate (OLR) of 2.16 kg COD/m3/d, the average COD removal efficiency was 71.5 ± 2.3%. Completely granular sludge can be observed after 107 d of continuous operation. Analysis of the distribution and composition of the extracellular polymeric substances (EPS) indicates that the tightly bound EPS (TB-EPS) content shows an increasing trend, while the loosely bound EPS (LB-EPS) content did not significantly alter after the granular sludge was formed. The three-dimensional excitation-emission matrix technique (3D-EEM) confirms that aromatic protein-like substances are of key importance to sludge granulation. High-throughput sequencing analysis indicates that the metabolism shifted from hydrogenotrophic (Methanobaterium) to aceticlastic methanogens (Methanosaeta) during sludge granulation.

Influence of CeO2 nanoparticles on viscoelastic properties of sludge: Role of extracellular polymeric substances

Cerium oxide nanoparticles (CeO2 NPs) affected the production of extracellular polymeric substances (EPSs), and thus might bring challenges for sludge pumping and mixing. In the present study, we investigated the rheological behavior of sludge before and after extraction of different EPSs fractions under various CeO2 NPs concentrations. It was found that the removal of loosely bound EPSs (LB-EPSs) could affect the shear stress (τ) and apparent viscosity (η), and the changes were dependent on CeO2 NPs concentrations. The removal of tightly bound EPSs (TB-EPSs) either with or without the addition of CeO2 NPs significantly decreased the yield stress (τy) and the limiting viscosity (η∞). Furthermore, the dynamic (strain, frequency and time) sweep measurements proved that the storage modulus (G') decreased after the extraction of TB-EPSs, indicating the weakened elastic and solid-like properties. The fluctuated content of polysaccharide in LB-EPSs and the increased amount of protein in TB-EPSs were likely to contribute to the variation of viscoelastic behaviors after the removal of LB-EPSs and TB-EPSs, respectively. In addition, the decreased rheological properties of sludge was also related to the increased zeta potential, decreased particle size and the removal of key organic matters of (104–106 Da) with the extraction of stratified EPSs. These results were significant to take advantages of the rheological properties for sludge treatment in the presence of NPs.

Application of sulfate radicals from ultrasonic activation: Disintegration of extracellular polymeric substances for enhanced anaerobic fermentation of sulfate-containing waste-activated sludge

Sulfate radicals produced by ultrasonic activation of persulfate (PS) or sulfate were applied to disintegrate sludge flocs and extracellular polymeric substances (EPSs). The pretreatment facilitated the production of volatile fatty acids (VFAs) and hydrogen from subsequent anaerobic fermentation, yielding a potential method for improving the utilization of sulfate-containing waste-activated sludge (WAS). Sulfate radicals produced using ultrasound (US) in the presence of PS/sulfate showed a synergetic effect of advanced oxidation on WAS pretreatment. Among the three types of pretreatment (US, US + Sulfate, and US + PS), US + PS pretreatment was the most favoured for chemical oxygen demand (COD) solubilization, protein release, and sludge disintegration under a US density of 1.5 W/mL. Tightly bound EPS (TB-EPS) could be effectively disintegrated, and organic compounds were transformed into loosely bound EPS and soluble EPS. Unexpected COD consumption by excessive free radicals was also observed, when increasing the ultrasonic energy input (e.g. to 12 W/mL), towards a positive role for sludge disintegration and subsequent anaerobic fermentation. It was found that small particles (0.1–0.5 μm) from released organics could be preferentially overoxidized. An appropriate low ultrasonic strength initiated attack on the polymeric substances in WAS by the produced radicals, which TB-EPS could be primary target.

Effect of using different proportions of inoculum during bioleaching on sludge dewaterability.

Bioleaching, the addition of bacteria to geological materials, has been applied to sludge to remove metals and improve upon sludge dewaterability. This paper investigates the effect of using different quantities of inoculum (bacteria) during bioleaching on sludge dewaterability. The analysis was based on bioleaching experiments conducted in a 20 L bio-reactor using different quantities of inoculum (20%, 10%, 5%, 2%, 0%). Changes in pH, oxidation reduction potential (ORP), capillary suction time (CST), specific resistance to filtration (SRF) and extracellular polymeric substances (EPS) were determined to gauge sludge dewatering. Results indicate that sludge dewaterability during the 2%, 10%, and 20% inoculum experiments declined through time. Decreased dewaterability is attributed to increases in the quantity of proteins and polysaccharides in slime EPS. Dewaterability improved during the 5% inoculum experiment, and reached a maximum when pH was 2.3. During this latter experiment, CST and SRF were reduced by 74% and 62%, respectively, in comparison to control conditions, while total EPS content decreased by 71%. The decrease in total EPS was primarily due to a decrease in proteins associated with tightly bound EPS (TB-EPS). Thus, changes in the amount of proteins in TB-EPS and sludge pH played a crucial role in sludge dewaterability.

Anammox granular sludge in low-ammonium sewage treatment: Not bigger size driving better performance

An integrated investigation to document high anammox abundance, activity and diversity in upflow anaerobic sludge blanket (UASB) reactor treating low-strength ammonium loading sewage was performed and showed that the optimal anammox granular sludge sizes could mitigate undesirable N2O emission. The enhanced anammox bacterial abundance, activity and specific anammox rate were achieved with optimal granules sludge sizes of 0.5–0.9 mm with multiple “Jettenia”, “Brocadia”, and “Anammoxoglobus” species. The tightly-bound extracellular polymeric substance (TB-EPS) was the main EPS layer found in anammox granular sludge, in which polysaccharides play an important structural role. Over this granular sludge sizes, the anammox bacterial abundance and activity did not significantly decrease, but N2O emission significantly increased. High throughput sequencing and ecological networks demonstrated the patterns of anammox and their co-occurring bacteria, with availability N2O-producer and N2O-reducer functional genes. Incomplete denitrification and insufficient carbon source mainly contributed to N2O production in granular sludge, as supported by results of stratification analysis.

Flocculation of microalgae using extracellular polymeric substances (EPS) extracted from activated sludge

This study investigates the role of microbial extracellular polymeric substances (EPSs) as bioflocculants to harvest microalgae (water-microalgae separation). The EPS extracted from waste activated sludge (WAS) by heat extraction were fractionated into soluble EPS (S-EPS), loosely-bound EPS (LB-EPS) and tightly-bound EPS (TB-EPS) forms. All the EPSs facilitated the flocculation of microalgal cells from stable growth medium. Of those EPSs, the TB-EPS showed the highest flocculating activity (FA) resulting in the substantial decrease in the amount of EPS added in terms of total organic carbon (TOC) during flocculation. The FA of microalgae was improved with the increase in TB-EPS dose, however, excessive dose of TB-EPS adversely affected it due to destabilization. Both LB- and TB-EPS could be utilized for flocculating microalgae as a sustainable option to the existing chemical-based flocculants. In addition to the conventional assessments, the effectiveness of the two bioflocculants for floc forming was also confirmed using a novel assessment of lens-free shadow imaging technique (LSIT), which was firstly applied for the rapid and quantitative assessment of microalgal flocculation.

Citric acid assisted Fenton-like process for enhanced dewaterability of waste activated sludge with in-situ generation of hydrogen peroxide

Fenton's reagent has been widely used to enhance sludge dewaterability. However, drawbacks associated with hydrogen peroxide (H2O2) in Fenton's reagents exist, since it is a hazardous chemical and shows carcinogenicity, explosivity, instability, and corrosivity. Moreover, initial acidification and subsequent neutralization are needed as optimal conditions for homogeneous Fenton conditioning and final filtrate discharge. In this study, a Fenton-like process for the enhanced dewaterability of waste activated sludge with in-situ generation of H2O2 and without extra pH adjustment was firstly proposed, namely citric acid (CA)-assisted oxygen activation in an air/nano zero-valent iron (nZVI) system and chemical re-coagulation with polydiallyldimethylammonium chloride (PDMDAAC). Using the response surface methodology (RSM), the optimal doses of CA, nZVI, and PDMDAAC were determined to be 13, 33, and 9 mg g−1 dry solids (DS), respectively. This composite conditioner showed a good dewatering capability compared with the raw sludge, e.g. the capillary suction time decreased from 130.0 to 9.5 s. The enhanced sludge dewaterability was further confirmed by laboratory-scale diaphragm filter press dewatering tests, which produced a lower cake moisture content compared with the raw sludge, and the final pH of the filtrate was close to neutrality. The citric acid promoted the production of H2O2 and Fe(II)/Fe(III) species, the degradation of protein in tightly-bound extracellular polymeric substances, and the decomposition of protein-N in the solid phase of sludge, resulting a greater conversion of bound water to free water. The results of electron spin resonance indicated that the hydroxyl radicals were mainly responsible for the decomposition of proteinaceous compounds. The subsequent chemical re-coagulation with PDMDAAC can make the zeta potential of sludge samples less negative, reduce the repulsive electrostatic interactions, and agglomerate the smaller particles into larger aggregates, thus enhancing sludge dewaterability.

Impact of recharge water temperature on bioclogging during managed aquifer recharge: a laboratory study

To investigate the effect of recharge water temperature on bioclogging processes and mechanisms during seasonal managed aquifer recharge (MAR), two groups of laboratory percolation experiments were conducted: a winter test and a summer test. The temperatures were controlled at ~5±2 and ~15±3 °C, and the tests involved bacterial inoculums acquired from well water during March 2014 and August 2015, for the winter and summer tests, respectively. The results indicated that the sand columns clogged ~10 times faster in the summer test due to a 10-fold larger bacterial growth rate. The maximum concentrations of total extracellular polymeric substances (EPS) in the winter test were approximately twice those in the summer test, primarily caused by a ~200 μg/g sand increase of both loosely bound EPS (LB-EPS) and tightly bound EPS (TB-EPS). In the first half of the experimental period, the accumulation of bacteria cells and EPS production induced rapid bioclogging in both the winter and summer tests. Afterward, increasing bacterial growth dominated the bioclogging in the summer test, while the accumulation of LB-EPS led to further bioclogging in the winter test. The biological analysis determined that the dominant bacteria in experiments for both seasons were different and the bacterial community diversity was ~50% higher in the winter test than that for summer. The seasonal inoculums could lead to differences in the bacterial community structure and diversity, while recharge water temperature was considered to be a major factor influencing the bacterial growth rate and metabolism behavior during the seasonal bioclogging process.

Effect of wastewater particles on catalytic ozonation in the advanced treatment of petrochemical secondary effluent

The effect of wastewater particles on catalytic ozonation during the treatment of petrochemical secondary effluent (PSE) was investigated. The dissolved organic carbon (DOC) removal results indicated that wastewater particles could significantly affect the catalytic oxidation reaction and increase ozone (O3) consumption. When the suspended solids (SS) was filtered by the 0.45 μm membrane, the O3 dosage to DOC removal ratio was 1.99 mgO3 mgDOC−1, while it increased to 2.99 mgO3 mgDOC−1 when the SS concentration was 50 ± 1.57 mg L−1. Wastewater particles can react with O3, thus reducing the O3 and hydroxyl radicals (OH) available for DOC removal. When the SS concentration was 30 ± 0.96 mg L−1, O3 dosages lower than 10 mg L−1 did not induce a change in the size and surface characteristics of the particles. Following the increase in the O3 dosage to 36 mg L−1, extracellular polymeric substances (EPS) reacted with O3/OH with different characteristics. Dissolved organic matter (DOM) had the highest depletion reaction rate, followed by the loosely bound EPS (LB-EPS) and tightly bound EPS (TB-EPS). The average reaction rates of the three constituents were 1.44, 0.212, and 0.079 mg L−1 min−1, respectively. In addition, the results showed that eliminating wastewater particles would be beneficial for the removal of organic micro-pollutants (OMPs) by catalytic ozonation.

Combining microwave irradiation with sodium citrate addition improves the pre-treatment on anaerobic digestion of excess sewage sludge

This study investigated the synergistic effect of sodium citrate (SC; Na3C3H5O(COO)3) and microwave (MW) treatment on the efficiency of the anaerobic digestion of excess sewage sludge. In terms of the methane yield, an increase of the digestion's efficiency was observed. Taking into account the cost for the MW energy supplied to the system, the optimum treatment conditions were a MW energy input of 20 MJ/kg TS and a SC concentration of 0.11 g/g TS, obtaining a methane yield of 218.88 ml/g VS, i.e., an increase of 147.7% compared to the control. MW treatment was found to break the sludge structure, thereby improving the release of extracellular polymeric substances (EPS) and volatile fatty acids (VFAs). The treatment of sodium citrate further strengthened the breakage of loosely bound extracellular polymeric substances (LB-EPS) and tightly bound extracellular polymeric substances (TB-EPS). The increased VFA content stressed the improved digestion by this pretreatment. Furthermore, the preliminary economic analysis showed that at this point in the research, only operational but no financial gains were achieved.

Effects of Magnetic Fe3O4 Nanoparticles on the Characteristics of Anaerobic Granular Sludge and Its Interior Microbial Community

In this study, the effects of magnetic Fe3O4 nanoparticles (Fe3O4 NPs) on soluble microbial products (SMP), loosely bound extracellular polymeric substances (LB-EPS), and tightly bound extracellular polymeric substances (TB-EPS) in anaerobic granular sludge were examined. In addition, the anaerobic granular sludge interior microbial community dynamics were investigated using high-throughput sequencing. The results demonstrated that the removal rate of COD was 83.6% after long-term exposure in the experimental reactor, namely, the anaerobic reactor containing Fe3O4 NPs. It was reduced by 5.7% in comparison with the removal rate in the control reactor. The total amount of TB-EPS in anaerobic granular sludge in the experimental and control reactors was 178.20 mg·g-1 and 138.24 mg·g-1, respectively, while the total amount of SMP in anaerobic granular sludge was 34.88 mg·L-1 and 27.44 mg·L-1, respectively. With regard to the LB-EPS in anaerobic granular sludge in the experimental reactor, the peak of humic acid disappeared and the peak intensity of coenzyme F420 decreased slightly using excitation-emission matrix (EEM) fluorescence spectra. In terms of the microbial community dynamics in the experimental reactor, the abundance of Methanobacterium was greatly augmented from 76.15% to 86.76%; whereas, the abundance of Methanothrix decreased from 17.1% to 7.51%. This indicated that Methanothrix was more sensitive to Fe3O4 NPs. Moreover, the changes in bacterial communities were evident:①the abundance of Proteobacteria dropped from 66.44% to 47.16%; ② the abundance of Actinobacteria grew from 8.97% to 17.33%; and ③ the abundance of Bacteroidetes increased from 8.07% to 17.74%. The increasing abundance of Actinobacteria and Bacteroidetes plays a positive role in the anaerobic hydrolysis of organic matter.

Characterization of a new continuous gas-mixing sulfidogenic anaerobic bioreactor: Hydrodynamics and sludge granulation

Continuous gas recirculation (CGR) was demonstrated in this study to be an effective method to mitigate the persistent problem of sludge flotation in high-rate sulfate-reducing upflow sludge bed (SRUSB) reactors that do not produce much gas. The effects of hydraulic- and CGR-mixing on the mixing regime of the SRUSB reactors were investigated over a period of 45 d at the average shear rates of 0.9, 1.5, 2.7, 4.2 and 7.2 s−1 (Phase I). CGR-mixing at 4.2 s−1 resulted in the smallest reactor short-circuiting flow of 1.3 ± 0.1% and the smallest dead zone volume of 0.2 ± 0.01% at a lower power consumption (0.0007 W) than hydraulic mixing. In Phase II, the SRUSB reactor with CGR-mixing at 4.2 s−1 was re-inoculated and operated for 150 days. Within the first 65 days, the sludge transformed into micro-granules (300–350 μm) with a high sulfate-reducing bacteria (SRB) activity (0.62 ± 0.05 g COD/(g MLVSS·day)), a low sludge flotation potential (<20%) and a high settleability (SVI5/SVI30 < 1.3). These results are attributed to the following sludge properties: (i) a low ratio of loosely-bound to tightly-bound extracellular polymeric substances (0.06–0.1), (ii) weakly adhesive surface properties as demonstrated by a strongly negative zeta potential (−23 ± 2 mV), a low hydrophobicity (37 ± 3%) and a low viscosity (0.7 ± 0.1 mPa s), and (iii) small size granules resulting in strong mass transfer (sulfate and COD penetration into the granule core) and a homogeneous structure (SRB detected throughout the granule).

Unraveling the catalyzing behaviors of different iron species (Fe2+ vs. Fe0) in activating persulfate-based oxidation process with implications to waste activated sludge dewaterability

Dewatering of waste activated sludge (WAS) is of major interest in its volume reduction, transportation and ultimate disposal. Persulfate-based oxidation process is a newly developed option for enhancing WAS dewaterability through the generation of powerful sulfate radicals (SO4−·). However, the enhancement in WAS dewaterability by persulfate differs with the species of iron catalysts used. In this study, two types of iron catalysts (i.e. Fe2+ vs. Fe0) were employed to initiate the persulfate (S2O82−), and the catalyzing behaviors and the underlying principles in enhancing WAS dewaterability were investigated and compared. The Fe2+ exhibited the high effectiveness in catalyzing the decomposition of persulfate to sulfate radicals (SO4−·), inducing the greater improvement in WAS dewatering. The WAS dewaterability (indicated by dry solids content after filtration) increased with the added S2O82−/Fe2+ dosages, with the dry solids content reaching up to 5.1 ± 0.8 wt% at S2O82−/Fe2+ dosages of 1.2/1.5 mmol/g-VS after only 30 s’ filtration, roughly 1.8-fold increase than raw WAS (1.8 ± 0.1 wt%). In contrast, the influence of the persulfate oxidation when activated with Fe0 on WAS dewaterability was statistically insignificant. The WAS dewaterability remained nearly unchanged (i.e. dry solids content of 2.0 ± 0.0 wt%), irrespective of the employed S2O82−/Fe0 dosages. Further analysis demonstrated that the WAS dewaterability negatively corresponded to loosely bound extracellular polymeric substances (LB-EPS) and tightly bound EPS (TB-EPS). The abundant SO4−· from S2O82−/Fe2+ system could effectively disrupt the gel-like EPS matrix, break apart the cells and subsequently arouse the release of the water inside EPS and cells, facilitating water-solid separation. In the case of S2O82−/Fe0, the dissolution of Fe0 particles was the rate-limiting step, due to the formation of oxide iron layer near Fe0 metallic surface, which resulted in the slow SO4−· production and thus hardly promoted WAS dewaterability. The pH adjustment could accelerate Fe0 dissolution and enhance the dewatering performance of S2O82−/Fe0 process to a certain degree, but the effect was unsatisfactory. Additionally, the observations regarding the dissolved organic matters and ammonium collectively revealed that except for enhancing WAS dewatering, S2O82−/Fe2+ oxidation could concurrently degrade COD and ammonia from WAS filtrate, lighten the burden of the subsequent sewage treatment facilities and reduce operational expense. Hence, from an environmental and economic perspective, the S2O82−/Fe2+ system possesses much greater promise for WAS dewatering.