Sludge Extracellular Polymeric Substances Research Articles

Effects of Light Energy with Different Densities on Denitrification: Metabolic Intermediates, Electron Behavior, Enzyme Activities, and Sludge Settleability

Light energy, as one of the most influencing parameters in microalgal–bacterial consortium (MBC) systems, can regulate activities and metabolisms of microalgae and bacteria consortia. However, its effects on denitrifying bacterial remain unclear. Herein, variations of denitrification, microbial (denitrifiers’) metabolism, sludge surface characteristics, and settleability were systematically investigated with five light energy density conditions. Results showed that light energy decreased the denitrification rate with obvious accumulation of denitrifying intermediates (NO2– and N2O). This could be associated with the inhibition of light energy on electron behavior, evidenced from reduced nicotinamide adenine dinucleotide (by 67.0–71.3%), lowered electron transport system activity (by 66.7%), decreased denitrifying enzyme activities (e.g., NAR, NIR, and NOS), and downregulated gene expressions. Moreover, light energy worsened sludge settleability by regulating sludge extracellular polymeric substance (EPS) types, compositions, and surface characteristics. Specifically, light energy stimulated the soluble and lightly bounded EPS, as well as the hydrophilic and electronegative functional groups in EPS. These findings advance our understandings on denitrifiers activities and their electronic behavior responses to short-term light energy. Ultimately, this may promote the development of MBC as a sustainable approach for wastewater treatment.

The coexistence of copper ions and TC affected the binding ability and the reaction order between extracellular polymeric substances of aerobic granular sludge and exogenous substances.

As a barrier against external toxic effects, extracellular polymeric substances (EPSs) directly affect the toxicity and removal efficiency of exogenous substances. The reaction of EPSs with exogenous substances has been taken into consideration. The contents of EPSs in sludge cultivated by different influent water vary greatly, which leads to great differences in the binding ability and reaction sequence between EPSs and exogenous substances. However, the results in this respect are very limited. In this study, the binding characteristics between exogenous tetracycline (TC)/copper ions (Cu2+) and EPSs from aerobic granular sludge cultured under single and coexisting TC/Cu2+ were assessed by three-dimensional fluorescence-parallel factor analysis. The pollutants in the influent water could directionally induce microorganisms to secrete more EPSs, while fluorescence substances in EPSs could combine with the exogenous substances to lessen their effects. In the presence of coexisting TC and Cu2+ in the influent water, the ability of fluorescence substances in EPSs to combine with exogenous TC or Cu2+ weakened, and humic substances in EPSs were more susceptible than protein substances to binding with exogenous substances. However, the reaction order between EPSs components and exogenous TC or Cu2+ was opposite, and the ability of fluorescence substances in EPSs to combine with exogenous TC or Cu2+ was enhanced under individual TC or Cu2+ existing in the influent water. This study provided new insights into the interaction between EPSs and exogenous substances.

Enhancing the dewaterability of waste activated sludge by the combined ascorbic acid and zero-valent iron/persulfate system.

In this work, a reducing/chelating agent, ascorbic acid (H2A) was introduced to the traditional zero-valent iron (Fe0)/persulfate (PS) process for waste activated sludge dewatering. The experimental data indicated that H2A-Fe0/PS process significantly enhanced the dewatering performance of sludge and enhanced the oxidation efficiency of Fe0-PS treatment. Under optimal conditions, the capillary suction time ratio before and after treatment (CST0/CST) of H2A-Fe0/PS treated sludge increased by 118% and 31.3% compared with untreated sludge and Fe0-PS treated sludge, respectively. The mechanism investigations revealed that the H2A-Fe0/PS induced excellent enhancement for sludge dewaterability could be credited to the reduction and chelating capacity of ascorbic acid. Free radicals including SO4•-, O2•- and •OH produced in the H2A-Fe0/PS process destroyed proteinaceous components and humic substances in sludge extracellular polymeric substances (EPS), thus reducing the negative charge and water holding capacity of sludge, improving the sludge rheological properties. As a result, the dewatering performance of sludge has been significantly improved.

Effects of hydrothermal treatment on organic compositions, structural properties, dewatering and biogas production of raw and digested sludge

The effects of hydrothermal treatment (HTT) under different temperatures and time (120 °C to 250 °C, 10 min to 60 min) on organic matter solubilization and structure changes of secondary sludge (SS) and digested sludge (DS), as well as downstream dewatering and anaerobic digestion were investigated. The results showed that organic matter solubilization increased significantly at 120 °C to 170 °C, then decreased at 200 °C to 250 °C. The organic matter solubilization during HTT showed no obvious difference for two sludge, but for the different organic components. The polysaccharides are easier to be dissolved than protein, which was manifested by the higher dissolution rate at low temperature. The protein was the main soluble component for both of hydrothermal SS and DS, which accounted for 44 % to 64 % of soluble chemical oxygen demand (SCOD). The decrease of residual extracellular polymeric substances (EPS) content and increase of N-acetylglucosamine and DNA concentrations indicated that sludge EPS and cell wall structure were damaged at 170 °C, which contributed to the high organic matter solubilization. Nitrogen balance and molecular weight distribution indicated the concentrations of soluble organic components were the combined result of dissolution and hydrolysis reaction. The hydrolysis and polymerization reaction were intensified at 170 °C to 250 °C, which was verified by the COD balance and molecular weight transformation. The hydrothermal time could further facilitate the organics dissolution and hydrolysis based on the effect of hydrothermal temperature. The EPS structure damage also contributed to the high percentage of free moisture, resulting in enhanced dewaterability. The highest methane production was 298.1 mL CH4/g VSadd for DS hydrothermally treated at 170 °C, which were 125 % and 9.8 % higher than SS and SS-HTT, respectively. This study provided an insight into the general mechanism of HTT and the application of different HTT and AD configurations.

The effects of chlortetracycline on anaerobic digestion of chicken manure and the role of extracellular polymeric substances

In this study, the process stability and the protective role of extracellular polymeric substances (EPS) were evaluated during the anaerobic digestion (AD) of chicken manure (CM) suffering chlortetracycline (CTC) concentrations from 0 mg/L to 100 mg/L in a continuous stirred tank reactor (CSTR) operated for 270 days. Results showed that during the phases with low levels of CTC (0, 1.25, and 2.5 mg/L), the volume biogas production rate increased from 0.77 ± 0.01 L/(L·d) to 1.15 ± 0.02 L/(L·d) with an obvious decrease in the concentration of VFAs from 2.57 ± 0.07 g/L to 1.17 ± 0.08 g/L. The reactor remained stable with continued operation as the CTC in the CM spiked to 5 mg/L and 10 mg/L, respectively. During the phases with high levels of CTC (25, 50, and 100 mg/L), despite a reduction in biogas production of 18%, 43% and 56%, respectively, compared to the steady stage, the reactor finally could maintain the inhibition-steady stage. This stage was characterized by steady biogas production along with accumulating VFAs and ammonia nitrogen, which increased the inhibition thresholds of CTC on the AD of CM. An appreciable increase in EPS was identified. Spectral analysis results showed the fluorescence of protein-like in EPS was quenched with increasing dosages of CTC. The quenching constant (Kq) induced by CTC for tyrosine was found to be 3.969 × 1012 (R2 = 0.942), implying that the increased CTC prompt anaerobic microbes excrete more EPS to protect themselves from the CTC toxins by the formation of the complex tyrosine-CTC. This protection led to a decrease in the biodegraded CTC from 61.6 to 40.9%, and an increase in the adsorbed CTC on EPS in sludge from 35 to 56.1%. The results indicated that the absorbed CTC on EPS may pose a risk to the soil ecosystem due to the agriculture utilization of the biogas residue.

Observing the Biologically Induced Phosphate Precipitation by Sludge Extracellular Polymeric Substances in Enhanced Biological Phosphorus Removal

Observing the Biologically Induced Phosphate Precipitation by Sludge Extracellular Polymeric Substances in Enhanced Biological Phosphorus Removal

Influence of composite fillers on sludge characteristics and extracellular polymeric substance composition and structure in the sequencing batch reactor

Influence of composite fillers on sludge characteristics and extracellular polymeric substance composition and structure in the sequencing batch reactor

Molecular spectroscopy and docking simulation revealed the binding mechanism of phenol onto anammox sludge extracellular polymeric substances

The rapid development of chemical industry has induced to the large amount of phenolic wastewater production. When the promising anaerobic ammonium oxidation (anammox) was employed to treat the industrial wastewater, phenolic compounds would possibly inhibit the microbial performance. Extracellular polymeric substances (EPSs) play an essential role in protecting cells from being intoxicated by phenolic compound while the distinct mechanism remains elusive. In this work, the interaction of phenol with anammox sludge EPSs and transmembrane ammonium transport (Amt) domain was explored at molecular level by using spectral method and molecular docking simulation. It was found that phenol statically quenched the fluorescent components of EPSs and the protein component dominated the interaction between EPSs and phenol. The overall interaction was an entropy-driven process with hydrophobic interaction as the main driving force, and the CO vibration responded preferentially. As phenol continued to penetrate into the cell surface, there were hydrogen bond, hydrophobic interaction force and π-π base-stacking forces between the Amt domain and phenol. The interaction between phenol and amino acid residues of the Amt domain would interfere the NH4+ transport and further affect the activity of anammox sludge. This work is beneficial for in-depth understanding the role of EPSs in protecting anammox sludge from inhibiting by phenolic pollutants.

Calcium Hypochlorite Promotes Dark Fermentative Hydrogen Production from Waste Activated Sludge

This study reported a high-efficiency and low-cost sludge pretreatment method using calcium hypochlorite [Ca(ClO)2] for promoting anaerobic fermentation performance. The experimental results indicated that the cumulative hydrogen yield was enhanced from 1.87 to 16.72 mL/g volatile suspended solids (VSSs) when the Ca(ClO)2 dosage was increased from 0 to 1.5 g/L and then reduced to 13.80 mL/g VSSs when the Ca(ClO)2 dosage reached 1.8 g/L. The mechanism study revealed that Ca(ClO)2 efficiently disrupted sludge extracellular polymeric substances and microbial cells, producing more soluble organics for the subsequent biochemical processes. Beyond that, the biodegradability of sludge organics was promoted by Ca(ClO)2 pretreatment. Biochemical reaction kinetic analysis indicated that the activities of microbes responsible for hydrogen consumption were severely inhibited when treated with Ca(ClO)2. The two decomposition products of Ca(ClO)2, Ca(OH)2 and HClO, positively affected the hydrogen yield, with the contribution of Ca(OH)2 being greater than HClO. Microbial community analysis illustrated that the fermentative microbes were markedly enriched by Ca(ClO)2 pretreatment, with the abundances found to be 3.21 and 45.62% in control and Ca(ClO)2-pretreated fermenters, respectively. After fermentation, the Ca(ClO)2-pretreated sludge showed better dewaterability than the control, and the organic pollutants diclofenac and sulfamethazine, as well as the fecal coliforms, were largely removed by Ca(ClO)2.

Effects of Waste Activated Sludge Extracellular Polymeric Substances on Biosorption

Extracellular polymeric substances (EPS) reportedly make up approximately half of the organic matter in activated sludge (AS), and therefore strongly influence AS properties. This study evaluated the component fractions of EPS normalized to volatile suspended solids (VSS) in waste activated sludge (WAS) from a trickling-filter-solids contact process (TF/SC) and its ability to biosorb organic matter from raw wastewater with 30 min of contact time. Biosorption is the process in which organic matter (carbohydrates, proteins, humic acids, DNA, uronic acids, and lipids) in a sorbate, such as raw wastewater, sorbs onto a sorbent such as WAS. A statistically significant correlation was found between both the total concentration of EPS and the proteins component of the EPS and the biosorption removal of soluble chemical oxygen demand (sCOD) and truly soluble COD (ffCOD). Thus, the biosorption of soluble forms of COD can accurately be predicted by quantifying just the amount of proteins in WAS-associated EPS. No significant correlations were found for the biosorption of colloidal COD (cCOD). WAS biosorbed 45–75 mg L−1 of COD in 30 min. WAS absorbed or stored the proteins fraction of the soluble microbial products (SMP) during the biosorption process. Higher concentrations of humic acids were found in the biosorption process effluent than in the untreated wastewater, which warrants further study. Longer cation exchange resin (CER) extraction times yielded more total EPS from the sludge: 90 ± 9, 158 ± 3, and 316 ± 44 mg g−1 VSS, for 45-min, 4-h, and 24-h extraction times, respectively. Thus, EPS extracted represented only 9%, 15.8%, and 31.6% of the VSS, respectively, raising questions about whether the accurate characterization of EPS can be performed using the typical extraction time of 45 min due to different extraction rates for different components. It was found that the humic acids fraction was extracted much more slowly than the other fractions.

Chemical extraction methods for activated sludge extracellular polymeric substances (EPS): A review

Waste activated sludge (WAS) is formed by a diverse microorganisms, organic and inorganic compounds merged and mixed together in an extracellular polymeric substances (EPS) network. EPS is a complex high-molecular weight macromolecules in WAS that happens to be one of the common analysis to determine the efficiency of treatment. Therefore, different extraction methods has been applied in order to achieve better EPS extraction yield. This paper serves as a base to review the commonly used chemical extraction methods to extract EPS components. The mechanisms, conditions and efficiencies of each of the chemical extraction methods were discussed and compared accordingly. The possible use of different chemical extraction methods for different type of activated sludge were summarized.

Improvement of sludge dewaterability by energy uncoupling combined with chemical re-flocculation: Reconstruction of floc, distribution of extracellular polymeric substances, and structure change of proteins

This study innovatively combines energy uncoupling and chemical re-flocculation helped to accelerate residual sludge dewatering. Ferric chloride (FeCl3) and 3, 3′, 4′, 5-tetrachlor-osalicylanilide (TCS) were employed as the flocculant and uncoupler, respectively. The results showed that the specific resistance to filtration (SRF) and the water content of sludge filtered cake fell dramatically from 11 × 1012 m/kg and 80.2% to 1.1 × 1012 m/kg and 77.1% respectively, when the addition of TCS ranged from 0 to 0.12 g/g VSS with flocculation conditioning. The distribution of sludge extracellular polymeric substance (EPS) was altered radically after adding TCS, leading to the collapse and fragmentation of EPS, causing the reduction and formation of fragmentized sludge flocs. Meanwhile, the stretching and deformation vibrations of CO and NH bonds suggested the strong attack between TCS and EPS proteins, while variations of the main secondary structures of protein (i.e. α-helix, β-sheet and random coil) indicated the loose structure of proteins and enhanced hydrophobicity. Consequently, the cracked and loose structure of residual sludge resulted in the release of bound water. After TCS addition combined with chemical re-flocculation, the channels of sludge water discharge were widened, guaranteeing the discharge of sludge water. Therefore, the sludge dewaterability was elevated under the energy uncoupling combined with chemical re-flocculation. As well, the application of TCS would not destroy sludge cells, in which bioenergy (sludge carbon source) could be retained and effectively utilized in the subsequent disposal process. The findings reported here not only widen our perception of the energy uncoupling technology, but also encourage researchers to explore both effective and economic methods on the basis of energy uncoupling, aiming to achieve high-efficiency of reduction and dewatering in the future.

Changes in molecular structure of extracellular polymeric substances (EPS) with temperature in relation to sludge macro-physical properties

Extracellular polymeric substances (EPS) are important components of activated sludge, whose content and composition have important effects on the macro-physical properties of sludge. In this study, the response of EPS in sludge to temperature (−40–200 °C) was systematically investigated using XAD resin fractionation, variable-temperature infrared spectra (VTIS) and two-dimensional correlation spectroscopy (2D-COS). The relationships between the molecular structure of EPS and the macro-physical properties (rheological property and dewatering performance) of waste activated sludge (WAS) at varying temperature were also established. During the freezing treatment, the solubilization of biopolymers and destruction of the hydrophilic functional groups (hydroxy, amino and carboxyl) resulted in the production of small organic matters, which enhanced EPS hydrophobicity and reduced electrostatic repulsion of sludge, and subsequent dewaterability improvement. For the hydrothermal treatment, the EPS transformation showed a two-stages reaction including stage I (70–120 °C) and stage II (>120 °C). Stage I (70–120 °C), a plenty of hydrophilic functional groups (hydroxy, amino and carboxyl) in EPS were exposed via the solubilization of biopolymers, which enhanced electrostatic repulsion of sludge and EPS hydrophilicity, and subsequence in deterioration of sludge dewaterability and fluidity. However, at stage II (>120 °C), the high temperature caused hydrolyzation of macromolecular organic matters in completely, in which the secondary structure of the protein was destroyed, causing the peptide chain to unfold. In addition, the reduction of α-helix and β-sheet content and intensified Maillard reaction decreased electrostatic repulsion of sludge, thus resulted in the improvement of sludge dewaterability and fluidity. This study enriched the theoretical basis of the optimal control of sludge treatment based on temperature regulation.

Freezing pretreatment assists potassium ferrate to promote hydrogen production from anaerobic fermentation of waste activated sludge

Anaerobic fermentation is an eco-friendly technology for waste activated sludge (WAS) treatment, during which resource recycle can be achieved. However, traditional sludge anaerobic fermentation is limited by the poor efficiency. We herein reported a novel high-efficiency technology by combining freezing with potassium ferrate (PF) for sludge pretreatment to promote hydrogen production from anaerobic fermentation. Experimental results demonstrated that freezing coupled with PF pretreatment exerted positively synergetic effect on hydrogen production. The maximal hydrogen production of 12.50 mL/g VSS (volatile suspended solids) was detected in the fermenter pretreated by freezing (−12 °C for 24 h) coupled with PF at 0.15 g/g TSS (total suspended solids), which was 1.34, 2.33, and 7.91 times of that from the individual PF, individual freezing, and control fermenters, respectively. The simulation results based on the modified Gompertz model indicated that both the hydrogen production potential and rate were promoted by freezing coupled with 0.15 g/g TSS PF pretreatment, from 2.14 to 13.52 mL/g VSS and 0.012 to 0.163 mL/g VSS/h, respectively. Thorough mechanism investigations revealed that the sludge EPS (extracellular polymeric substances) and microbial cells were both effectively damaged by combined freezing and PF pretreatment, resulting in the acceleration of sludge disintegration. Further investigations demonstrated that except for the acidogenesis, the other biochemical processes were all inhibited by freezing coupled with PF pretreatment, but the inhibitory extent for hydrogen consuming processes was more serious than that responsible for its generation. Gene sequencing analysis illuminated that both of the hydrolytic and hydrogen generating bacteria were largely enriched in the combined pretreatment fermenter. Moreover, the dewatering performances of fermented sludge were found to be notably enhanced by freezing coupled with PF pretreatment.

Recovered granular sludge extracellular polymeric substances as carrier for bioaugmentation of granular sludge reactor

An increasing amount of industrial chemicals are being released into wastewater collection systems and indigenous microbial communities in treatment plants are not always effective for their removal. In this work, extracellular polymeric substances (EPS) recovered from aerobic granular sludge (AGS) were used as a natural carrier to immobilize a specific microbial strain, Rhodococcus sp. FP1, able to degrade 2-fluorophenol (2-FP). The produced EPS granules exhibited a 2-FP degrading ability of 100% in batch assays, retaining their original activity after up to 2-months storage. Furthermore, EPS granules were added to an AGS reactor intermittently fed with saline wastewater containing 2-FP. Degradation of 2-FP and stoichiometric fluorine release occurred 8 and 35 days after bioaugmentation, respectively. Chemical oxygen demand removal was not significantly impaired by 2-FP or salinity loads. Nutrients removal was impaired by 2-FP load, but after bioaugmentation, the phosphate and ammonium removal efficiency improved from 14 to 46% and from 25 to 42%, respectively. After 2-FP feeding ceased, at low/moderate salinity (0.6–6.0 g L−1 NaCl), ammonium removal was completely restored, and phosphate removal efficiency increased. After bioaugmentation, 11 bacteria isolated from AGS were able to degrade 2-FP, indicating that horizontal gene transfer could have occurred in the reactor. The improvement of bioreactor performance after bioaugmentation with EPS immobilized bacteria and the maintenance of cell viability through storage are the main advantages of the use of this natural microbial carrier for bioaugmentation, which can benefit wastewater treatment processes.

The extent of sludge solubilization allows to estimate the efficacy of ozonation for removal of polycyclic aromatic hydrocarbons (PAHs) in municipal sewage sludge

Ozone treatment has been proven as an effective technology for removing PAHs in municipal sewage sludge. However, given the complex interaction of PAHs with sludge extracellular polymeric substances (EPS), effective sludge disintegration is required to make PAHs more accessible to oxidants, which also affects PAHs partitioning. Here, we investigated two treatment systems, namely ozonation (ozone) and sequential hydrodynamic cavitation and ozonation (HC+ozone), under varied conditions, to determine whether the extent of sludge disintegration can estimate a measurable removal of PAHs. The results showed that both the PAHs removal efficiency and kinetics highly depended on pH and ozone dose. Although no significant removal of PAHs occurred during HC treatment, the observed rate constants for Σ16 PAHs removal in HC+ozone system were 1.2–1.4-fold higher than those obtained in ozone system under the same condition. The accelerated PAHs removal could be attributed to the reduced flocs size (80% smaller) and increased soluble EPS concentration (5-fold higher) with HC pretreatment. Lastly, a positive correlation was noted between the PAHs removal efficiency and soluble EPS concentration (R2 = 0.85), indicating the important role of dissolved and colloidal matter (DCM) for PAHs partitioning between aqueous and particulate phases.

Study and Treatment of Medical Liquid Waste, Raipur Chhattisgarh, India

Hospitals discharge considerable amounts of chemicals and microbial agents in their wastewaters. Problem chemicals present in hospital wastewater belong to different groups, such as antibiotics, X-ray contrast agents, disinfectants and pharmaceuticals. Many of these chemical compounds resist normal wastewater treatment. They end up in surface waters where they can influence the aquatic ecosystem and interfere with the food chain. Humans are particularly exposed by the drinking water, produced from surface water. Microbial agents of special concern are multiresistant microbial strains. The latter are suspected to contribute to the spread of antibiotic resistance. In this paper, we will discuss the different approaches towards hospital wastewater treatment. The principle of uncoupling hospitals from public sewers warrants in depth evaluation by technologists and ecotoxicologists as well as public health specialists. A pilot-scale membrane bioreactor (MBR) was installed and operated for 75 days at laboratory. It was fed an influent directly from the hospital drainage collection system. To study pharmaceutics compounds toxicity with micro pollutant concentrations ranging from low ng /L to mg/L, toward activated sludge. Lab scale experiments were conducted and microscopic techniques as confocal laser scanning microscopy (CLSM). Samples were observed using confocal scanning laser microscopy to characterize the extracellular polymeric substances of sludge (EPS content). The study was focused on efficacy the membrane bioreactor in treatment the hospital effluent and the extra cellular polymeric substances (EPS) as indicator of bacteria sensitivity to toxic agents. Results suggested that after 20 days of exposure the pharmaceuticals compounds induced a significant increase of concentration soluble EPS in flocs, the pharmaceuticals compounds not inhibited COD removal and the nitrification during the experiments. These findings are in agreement with the microscopic studies, which showed a significant increase of concentration EPS. The presence of the pharmaceuticals compounds and its main metabolites stimulates mechanisms of protection and production of EPS with a slightly higher production of proteins. The removal of PPCPs by biological treatment processes including membrane bioreactor (MBR) was studied during the experience. The performance of MBR was demonstrated to be stable after 20 days of the experience. Although presence the fouling, the membrane look as technique very important for elimination the organics micro pollutants from the hospital wastewater.

Particle size-dependent behavior of redox-active biochar to promote anaerobic ammonium oxidation (anammox)

Nitrate (NO3−) accumulation in effluent decreases the biological nitrogen performance of anammox process for wastewater treatment. Redox-active biochar has been demonstrated as an effective additive to improve microbial NO3− reduction via redox cycling of the surface functional groups. However, their redox reactivity toward anammox consortia remains unknown. In this study, we investigated the effects of corn stover-derived biochars with different particle sizes (10–30 μm (SP), 200–500 μm (MP) and 2–5 mm (LP)) on nitrogen removal performance of anammox reactors under continuous operation with varying nitrogen loading rates over the long term. The average nitrogen removal efficiencies of anammox reactors amended with SP-, MP- and LP-biochar were 97.9%, 97.5% and 87.9%, respectively, which were significantly higher than that of the control (80.5%). Effluent NO3− concentration was reduced by 40.7–45.2% with presence of biochar as compared to the control, which may be attributed to microbial interaction with Chloroflexi-affiliated heterotrophic denitrifying bacteria. The electrochemical analysis of the sludge extracellular polymeric substances (EPS) based on cyclic voltammetry and electrochemical impedance spectrum revealed that the extent of biochar to increase the electron-transport ability of EPS decreased with the biochar particle size (SP > MP > LP). Furthermore, SP-biochar outperformed LP-biochar for promoting the proliferation of anammox bacteria Candidatus Brocadia and increasing the abundance of anammox-associated functional genes (hzsA, hdh, nirS and napA) in the sludge. Together, these findings showed the stimulatory effect of redox-active biochar for anammox consortia and the particle size-dependence, which may potentially assist in operating anammox processes with high nitrogen removal performance.

Understanding synergistic mechanisms of ferrous iron activated sulfite oxidation and organic polymer flocculation for enhancing wastewater sludge dewaterability

The bound water in waste activated sludge (WAS) is trapped in extracellular polymeric substances (EPS) in the form of gel-like structure, leading to a great challenge in the sludge deep dewatering. Traditional flocculation conditioning is unable to destroy EPS and ineffective to remove the bound water in WAS. In this study, we employed integration of Fe(II)-sulfite oxidation and polyacrylamide flocculation (F/S-PAM) treatment for removing the bound water and improving sludge dewaterability under aerobic conditions. Meanwhile, the floc microstructure and EPS properties were examined to understand the mechanisms of F/S-PAM conditioning. F/S produced SO3·− radicals which could decompose the EPS in sludge, releasing bound water into free water. In addition, the formed Fe(III) from F/S led to re-coagulation of decomposed EPS, and C=O groups of tryptophan played the leading role in Fe-EPS association binding, causing transformation of the secondary structure of proteins (especially β-sheets and α-helices). Then, the introduction of PAM caused re-flocculation of disintegrated sludge flocs, enhancing the sludge filterability. This work provides a novel and cost-effective method for efficient removal of bound water in sludge, and subsequence improvement in sludge dewaterability.

Structural changes in model compounds of sludge extracellular polymeric substances caused by exposure to free nitrous acid

Previous studies demonstrate that free nitrous acid (FNA i.e. HNO2) detaches sewer biofilms, breaks down flocs of waste activated sludge (WAS) and enhances biogas production from WAS. This suggests possible interactions of FNA with organic extracellular polymeric substances (EPS) that bind the cells into biofilms or sludge flocs. This study evaluates the chemical interactions and reaction mechanisms between FNA and molecules representative of key EPS in biofilm and sludge flocs. Molecules chosen to represent components found in the extracellular polymeric matrix were treated with FNA at 6.09 mgN/L (NO2− = 250 mgN/L, pH = 5.0 ± 0.2, T = 22 °C) for 24 hours (conditions typically used in applications) so as to consider the hypothesized chemical interactions and the consequent reaction pathways. A number of analytical techniques were employed to measure the molecular changes in the EPS molecules including; proton (1H) nuclear magnetic resonance spectroscopy (NMR), electrospray ionisation mass spectrometry (ESI-MS) and gel permeation chromatography (GPC). The results demonstrated that FNA broke down a range of large EPS molecules including carbohydrates, protein and lipids to smaller molecules. Two mechanistic pathways have been proposed including electrophilic substitution, whereby the nitrosium ion (NO+) was the reactive electrophile, and oxidative radical reactions, through which the nitrogen radicals (.NO2, .NO) and reactive nitrogen intermediates (RNIs) (e.g. N2O3 and N2O4) formed from the decomposition of FNA became part of the reaction products. Larger, more complex organic molecules such as humic acid, required higher concentrations of FNA (6.09 mgN/L or greater) to cause molecular breakdown, whereas smaller molecules, such as calcium alginate, was broken down at lower concentrations (3.04 mgN/L). The study contributes to the understanding of the fundamental mechanisms behind the application of FNA for biofilm control and flocular sludge disintegration.