Synergetic conditioning via oxalic acid enhanced Fe2+/CaO2 and skeleton construct to achieve deep dewatering of sewage sludge

Extracellular polymeric substance (EPS) with highly hydrophilic groups and sludge with high compressibility are determined sludge dewaterability. Herein, Fe2+ catalyzed calcium peroxide (CaO2) assisted by oxalic acid (OA) Fenton-like process combined with coal slime was applied to improve sludge dewaterability. Results demonstrated that the sludge treated by 0.45/1/1.1-OA/Fe2+/CaO2 mM/g DS, the water content (WC), specific resistance to filtration and capillary suction time dropped to 53.01%, 24.3 s and 1.2 × 1012 m/kg, respectively. Under coal slime ratio as 0.6, WC and compressibility were further reduced to 42.72% and 0.66, respectively. The hydroxyl radicals generated by OA/Fe2+/CaO2 under near-neutral pH layer by layer collapsed EPS, resulting in the degradation and migration of inner releasing components and the exposure of inner sludge flocs skeleton. The hydrophilic tryptophan-like protein of TB-EPS were degraded into aromatic protein of S-EPS and exposed inner hydrophobic sites. The protein secondary structures were transformed by destroying hydrophilic functional groups, which were attributed to the reducing α-helix ratio and reconstructing β-sheet. Moreover, coal slime as the skeleton builder lowered compressibility and formed more macropores to increase the filterability of pre-oxidized sludge for the higher intensity of rigid substances. This study deepened the understanding of OA enhanced Fenton-like system effects on sludge dewaterability and proposed a cost-effective and synergistic waste treatment strategy in sludge dewatering.

Sustained oxidation of Tea-Fe(III)/H2O2 simultaneously achieves sludge reduction and carbamazepine removal: The crucial role of EPS regulation

Establishing an economic and sustained Fenton oxidation system to enhance sludge dewaterability and carbamazepine (CBZ) removal rate is a crucial path to simultaneously achieve sludge reduction and harmless. Leveraging the principles akin to "tea making", we harnessed tea waste to continually release tea polyphenols (TP), thus effectively maintaining high level of oxidation efficiency through the sustained Fenton reaction. The results illustrated that the incorporation of tea waste yielded more favorable outcomes in terms of water content reduction and CBZ removal compared to direct TP addition within the Fe(III)/hydrogen peroxide (H2O2) system. Concomitantly, this process mainly generated hydroxyl radical (•OH) via three oxidation pathways, effectively altering the properties of extracellular polymeric substances (EPS) and promoting the degradation of CBZ from the sludge mixture. The interval addition of Fe(III) and H2O2 heightened extracellular oxidation efficacy, promoting the desorption and removal of CBZ. The degradation of EPS prompted the transformation of bound water to free water, while the formation of larger channels drove the discharge of water. This work achieved the concept of treating waste with waste through using tea waste to treat sludge, meanwhile, can provide ideas for subsequent sludge harmless disposal.

Sludge dewaterability improvement with microbial fuel cell powered electro-Fenton system (MFCⓅEFs): Performance and mechanisms

Throughout the entire process of sludge treatment and disposal, it is crucial to explore stable and efficient techniques to improve sludge dewaterability, which can facilitate subsequent resource utilization and space and cost savings. Traditional Fenton oxidation has been widely researched to enhance the performance of sludge dewaterability, which was limited by the additional energy input and the instabilities of Fe2+ and H2O2. To reduce the consumption of energy and chemicals and further break the rate-limiting step of the iron cycle, a novel and feasible method that constructed microbial fuel cell powered electro-Fenton systems (MFCⓅEFs) with ferrite and biochar electrode (MgFe2O4@BC/CF) was successfully demonstrated. The MFCⓅEFs with MgFe2O4@BC/CF electrode achieved specific resistance filtration and sludge cake water content of 2.52 × 1012 m/kg and 66.54 %. Cellular structure and extracellular polymeric substances (EPS) were disrupted, releasing partially bound water and destroying hydrophilic structures to facilitate sludge flocs aggregation, which was attributed to the oxidation of hydroxyl radicals. The consistent electron supply supplied by MFCⓅEFs and catalytically active sites on the surface of the multifunctional functional group electrode was responsible for producing more hydroxyl radicals and possessing a better oxidizing ability. The study provided an innovative process for sludge dewaterability improvement with high efficiency and low energy consumption, which presented new insights into the green treatment of sludge.

Triclocarban transformation and removal in sludge conditioning using chalcopyrite-triggered percarbonate treatment

Herein, a facile combination approach of chalcopyrite and sodium percarbonate (CuFeS2+ SPC) was established to augment both TCC removal efficiency and sludge dewatering. Results showed that utilizing the CuFeS2 dosage of 600 mg/g total solids (TS) under the optimal condition, along with the SPC dosage of 12.5 mg/g TS, an initial pH of 4.0, and a reaction duration of 40 min, led to a substantial reduction of 53.9% in the TCC content within the sludge, accompanied by a notable decrease of 36.9% in the water content. Compared to well-studied iron-based advanced oxidation processes, CuFeS2 + SPC treatment proved to be more cost-effective and environmentally friendly. Mechanistic findings demonstrated that •OH oxidation played a significant role in TCC removal, with O2•- and 1O2 acting as secondary factors. During the CuFeS2 + SPC process, the received •OH, O2•-, and 1O2 destroyed the main binding sites of extracellular polymeric substances to TCC, including tryptophan-like protein, amide, CO stretch, and -COO- functional groups. As a result, approximately 50% of TCC was partially degraded within the solid sludge phase after the attack of radicals. Meanwhile, the decreased macromolecular organic compounds in solid sludge attenuated the binding efficacy of TCC, giving rise to the transfer of partial TCC to the liquid phase. Ultimately, the TCC in sludge was successfully removed, and five transformation products were identified. This study significantly contributes to our understanding regarding TCC transformation and removal in the sludge conditioning process.

Characterization of melanoidins in thermal hydrolysis sludge and effects on dewatering performance

Thermal hydrolysis pretreatment (THP) of sludge can form the refractory brown melanoidins due to the occurrence of the Maillard reaction, which adversely involves the subsequent sludge anaerobic digestion (AD) process. However, details of the generation pattern of melanoidins and how they affect the sludge dewaterability remain largely unknown. This work aims to determine an approach to characterize and quantify the melanoidins created by THP of sludge. On this basis, the effect of melanoidins on sludge dewatering performance was revealed by adding synthetic melanoidins to the mixed sludge. Experimental results showed that three-dimensional fluorescence-region integration (3DEEM-FRI) could effectively distinguish melanoidins from other organic substances and achieve semi-quantitative characterization in sludge. The melanoidins significantly deteriorated the sludge dewaterability, and the lowest solids content of the filter cake (TS) was only 17.78% at the addition of 480 mg (g TS)-1, which was a drop of about 20% compared to the control group. The mechanism investigations indicated that the internal structure of sludge becoming particularly complicated and the opportunities for molecules to collide with each other enlarged because of the contribution of melanoidins, resulting in the increment of the sludge apparent viscosity and consistency coefficient (k), a decline of the flow behavior index (n) and a weakening of flowability. Melanoidins could capture massive water molecules and carry negative charges with the decrease of sludge particle size and zeta potential value, which enhanced the electrostatic repulsion between sludge particles and abated the flocculation ability, thus further aggravating the sludge dewatering performance.

Enhanced dewaterability of sewage sludge by grafted cationic lignin-based flocculants

Lignin-based flocculants are widely used for wastewater purification, but their application in sludge dewatering has not yet been documented. In this study, a novel cationic lignin-based flocculant named LS-g-CPA was prepared by grafting cationic polyacrylamide (CPA) synthesized from methacryloyloxy ethyltrimethyl ammonium chloride (DMC) and acrylamide (AM) onto sodium lignosulfonate (LS), and its roles and underlying mechanisms in sludge conditioning were investigated. The results showed that LS-g-CPA effectively improved the dewaterability of sludge, reducing the filtration resistance and filter cake moisture content of sludge from 0.61 ± 0.05 × 1012 m/kg to 0.14 ± 0.02 × 1012 m/kg and 85.64 ± 0.25 % to 76.84 ± 0.41 %, respectively. The dewatering performance of LS-g-CPA was positively correlated with the DMC/AM ratio. The quaternary ammonium groups brought by DMC disrupted the reticular structure of extracellular polymeric substances, exposing hydrophobic residues and releasing bound water. Nevertheless, the key to LS-g-CPA for improving sludge dewatering lies more in the amphoteric flocculant properties that enhance sludge flocculation and the octopus-type structure that provides good drainage channels. This study reveals that lignin-based flocculants are effective in improving the dewaterability of sludge, which provides direct evidence for their application in sludge dewatering.

Promoting dewatering efficiency of sludge by bioleaching coupling chemical flocculation

In recent years, bioleaching has emerged as a cost-effective technology for enhancing the dewaterability of sludge. However, the lengthy treatment time involved in sludge bioleaching processes limits daily treatment capacity for sludge. Here, a novel approach was developed through a short time of sludge bioleaching with A. ferrooxidans LX5 (A. f) and A. thiooxidans TS6 (A. t) followed by polyferric sulfate (PFS) flocculation (A. f + A. t + PFS). After 12.5 h of the A. f + A. t + PFS treatment (30% A. f, 10% A. t, 40 mg/g DS S0, 60 mg/g DS FeSO4•7H2O, and 120 mg/g DS PFS), the reduction efficiency of specific resistance to filtration (SRF) and sludge cake moisture content reached 94.0% and 11.6%, respectively, which were comparable to the results achieved through 24 h of completed bioleaching treatment. In pilot-scale applications, the mechanical dewatering performance was notably improved following A. f + A. t + PFS treatment, with the low moisture content of the treated sludge cake (∼59.2%). This study provides new insights into the A. f + A. t + PFS process and holds potential for developing efficient and promising sludge dewatering strategies in engineering application.

Enhanced disintegration mechanism of surplus activated sludge to improve dewatering by thermally activated persulfate oxidation under mild temperature

The dewatering treatment is an essential process for the treatment and disposal of surplus activated sludge (SAS), and improving sludge dewatering performance is still a challenge and has become a research hotspot in recent years. The oxidation and disintegration of bacterial cells and extracellular polymeric substances (EPS) by active radicals produced by advanced oxidation processes (AOPs) were extremely promising to achieve deep sludge dewatering. This paper systematically studied the efficiency and mechanism of thermally activated persulfate (TAP) oxidation technology to the improvement of SAS dewatering performance. The results showed that the relative filterability (CST0/CST) was increased 2.52 times with 2.0 mmol/g VSS potassium peroxydisulfate (PDS) at 80 °C in 90 min. Under this condition, the Zeta potential of SAS significantly decreased from - 14.8 to - 1.44 mV, while the average particle size (dp50) decreased from 52.981 to 48.259 μm. Thermal treatment disrupted the sludge structure to release large amounts of EPS including polysaccharides and protein. Meanwhile, the results of three-dimensional excitation-emission matrix (3D-EEM) fluorescence spectra showed that the TAP treatment could expedite the disintegration of sludge, facilitating the decrease of total EPS content and conversion of tightly bound EPS (TB-EPS) to loosely bound EPS (LB-EPS) and soluble EPS (S-EPS). The mechanism of TAP process to improve SAS dewatering performance was revealed, which could contribute to breaking the bottleneck of sludge depth dewatering and provide a theoretical and technical basis for its practical application.

Enhancement of sludge dewaterability by repeated inoculation of acidified sludge: Extracellular polymeric substances molecular structure and microbial community succession

Improving the dewatering performance of sewage sludge is of great scientific and engineering significance in the context of accelerated urbanization and increasingly strict environmental regulations. Acidified sludge (AS) can improve sludge dewatering performance, but the dewatering effect of repeated inoculation is unclear. The effects of long-term repeated inoculation of AS on the sludge dewaterability were investigated. The molecular structure and microbial community succession of extracellular polymeric substances (EPS) are emphasized. The results revealed that increasing the inoculation ratio of AS reduced the pH, absolute value of sludge zeta potential, and sludge particle size, and the decreasing trend was more evident with prolonging treatment time. Under the conditions of 30% and 50% AS inoculation, the dewatering performance of the sludge was significantly improved (p < 0.05). Compared with the raw sludge, the specific resistance of filtration (SRF) and capillary suction time of 30% inoculation were reduced by 64.3% and 50.1% after 30 cycles, respectively. Excluding loosely bound (LB)-EPS, soluble (S)-EPS and tightly bound (TB)-EPS exhibited a visible decrease, the protein in TB-EPS was significantly related to sludge dewaterability (p < 0.05). The fluorescent components of aromatic protein and fulvic acid-like substances in TB-EPS were significantly associated with SRF, with a correlation coefficient 0.99 (p < 0.05). Both the increase in the percentages of random coil and decrease in α-helix in TB-EPS contributed to improving dewaterability. Increasing Firmicutes and decreasing Chloroflexi levels improved the sludge dewatering capacity. Repeated inoculation did not disrupt the dewatering effect of AS rather increased the feasibility of the engineering application of AS. Considering the dewatering performance and cost synthetically, 30% AS inoculated ratio is feasible for practical applications.

Enhanced degradation of extracellular polymeric substances by yeast in activated sludge to achieve sludge reduction

Candida Tropicalis was used to improve the dewaterability of activated sludge (AS) and reduce its biomass by degrading EPS in AS. The protein, polysaccharide, and hydrophilic amino acids in EPS decreased by 54.50, 29.20, and 61.01%, respectively. Meanwhile, molecular weight distribution indicated that yeast degraded macromolecular organics into small molecular ones. The direct addition of yeast to AS was more conducive to EPS degradation. With the addition of 0.75 g/L of wet yeast cells and 24 h of aeration enhanced the dewaterability of AS. The CST and MLSS decreased by 24.44 and 10.51%, respectively. After 30 days of operation of lab-scale continuous SBRs, the CST and MLSS of AS were reduced by 6.37 ± 2.01 and 3.57 ± 0.52%, respectively. FTIR spectroscopy results showed that some hydrophilic functional groups were reduced. This study provides a new approach for the in-situ reduction of AS in wastewater treatment plant.

Enhancement of excess sludge dewatering by three-dimensional electro-Fenton process based on sludge biochar

Deep dewatering of waste activated sludge (WAS) is still a challenge due to high content of bound water and non-Newton fluid properties of sludge flocs. Electro-Fenton (EF) can enhance sludge dewaterability, however, low pH needed in homogeneous EF and fine flocs after EF conditioning influenced deep dewatering of sludge and the subsequent resource recovery disposal. In this study, a three dimension electro-Fenton (3D-EF) using Fe modified sludge biochar (Fe@SBC) as particle electrode, heterogeneous Fenton catalyst and skeleton builder for deep dewatering of sludge under neutral pH was proposed. Fe@SBC obtained at 800 °C exhibited high capacity of H₂O₂ electrogeneration and activation due to high conductivity and content of 2e-ORR selectivity functional groups. With promoted generation of H₂O₂ and hydroxyl radical (•OH), 3D-EF with Fe@SBC showed higher decomposition of bound extracellular polymeric substances (EPS) and disintegration of cells in sludge flocs, resulting in releasing bound and intracellular water into free water. Compared with EF, 3D-EF with Fe@SBC800 had higher ability in breaking macromolecules of protein and polysaccharide, as well as removing -COOH and -NH₂ groups in EPS, which could facilitate release of bound water trapped in EPS and self-coagulation of fine flocs. During subsequent filtering process, Fe@SBC could enhance sludge filterability as skeleton builder. A synergetic effect of strong oxidation and physical conditioning were proposed in 3D-EF sludge dewaterability with Fe@SBC, and the improved oxidation by Fe@SBC was supposed to play the major role.

Enhancing sludge dewatering efficiency through bioleaching facilitated by increasing reactive oxygen species

Bioleaching facilitated by iron-oxidizing bacteria is regarded as a promising sludge dewatering method due to excellent dewaterability and low cost. However, a two-days bioleaching time for sludge conditioning decreased its daily treatment capacity. In fact, Fe²⁺ easily reacts with O₂ to produce reactive oxygen species (ROS) with high oxidizing activity. Can bioleaching performed in Fe²⁺-rich system generate ROS to decompose sludge extracellular polymeric substances (EPS)? Here both contribution of ROS produced in bioleaching to improve sludge dewaterability and the increase of ROS content to shorten sludge bioleaching treatment time were investigated. The introduction of H₂O₂ in sludge bioleaching treatment (BS+H₂O₂) to increase ROS could simultaneously improve sludge dewaterability and decrease bioleaching time. Specific resistance to filtration (SRF) and capillary suction time (CST) reduction ratios (90.3% and 80.9%) in BS+H₂O₂ process were much higher than those in other processes after only 30 min reaction. Mechanisms of improving sludge dewaterability in BS+H₂O₂ mainly included ROS oxidation and Fe³⁺ flocculation by analysis of the contribution factors. These findings not only provide an effectively method to promote sludge dewatering efficiency of bioleaching, but also give new sights into the design of cost-efficient processes for improving the sludge dewatering.

Enhanced sludge dewaterability by ferrate/ferric chloride: The key role of Fe(IV) on the changes of EPS properties

The complex characteristics of extracellular polymeric substances (EPS) seriously affect the improvement of sludge dewaterability. Ferrate (Fe(VI))/ferric chloride (Fe(III)) was applied through its strong oxidability to effectively enhance sludge dewaterablity by changing the properties of EPS in this study. Results confirmed that water content (WC), specific resistance to filtration (SRF) and capillary suction time (CST) fell from 82.8 %, 9.3 × 10¹⁰ s²/g and 35.1 s to 76.1 %, 2.6 × 10¹⁰ s²/g and 16.2 s, respectively, when adding 12 mg Fe(VI)/g VSS and 12 mg Fe(III)/g VSS with the dosing interval of 5 min. Investigations of the mechanism strongly suggested that Fe(VI) was successfully catalyzed by Fe(III), promoting the generation of methyl phenyl sulfone (PMSO₂) and facilitating the electron transfer, with Fe(IV) having the major role in the oxidation process. Furthermore, sludge water-holding capacity and hydrophilicity waned after oxidation due to the destruction of EPS structure, which promoted the decrement of bound water to enhance the discharge of sludge water, so as to improve the efficiency of dewatering.

Enhanced paper sludge dewatering and in-depth mechanism by oxalic acid/Fe2+/persulfate process

As a typical advanced oxidation process, Fe²⁺-persulfate (PDS) oxidation technology has been widely and efficiently reported for enhancing sludge dewaterability. However, higher dosage of Fe²⁺ must be added, which will restrain the oxidation efficiency of Fe²⁺-PDS process. In this work, the oxalic acid (OA)/Fe²⁺-PDS process was studied to improve paper sludge dewatering. With the OA dosage of 6 μmol (g total solid (TS))^-1, Fe²⁺ dosage of 0.3 mmol (g TS)^-1, and PDS dosage of 0.6 mmol (g TS)^-1, sludge dewaterability was improved more efficiently. The specific resistance to filtration and water content of sludge cake were decreased by 70.7% and 8.0%, respectively. In comparison with Fe²⁺-PDS process, the viscosities of sludge suspension and supernatant were further reduced by 3.73% and 51.77%, respectively, and the contents of extracellular polymeric substances fractions were increased. The improved sludge dewaterability in OA/Fe²⁺-PDS process was mainly contributed by the synergistic effect of oxidative disintegration by free radicals and flocs re-flocculation, the contributions of which were the orders: metal cations > sulfate radical > hydroxyl radical. OA enhanced the efficient disintegration of sludge flocs, released more bound water, generated more Fe³⁺-oxalate complexes, and finally increased the sludge particle size significantly, forming a larger aggregation and obvious cracks. Additionally, the stabilization of several heavy metals was improved due to the chelating capacity of OA. These works will provide a novel approach for sludge dewatering in the PDS oxidation process.

A novel aquatic worm (Limnodrilus hoffmeisteri) conditioning method for enhancing sludge dewaterability by decreasing filamentous bacteria

In this study, a novel aquatic worm conditioning method was proposed to enhance sludge dewaterability by reducing filamentous bacteria. The optimal treatment time was 4 days and the optimal sludge concentration was 5000 mg/L. Under these conditions, the sludge dewaterability was improved with CST of 16.69 s, reduction in sludge SRF of 48.95 %, and reduction in LfA of 58.23 %. After bio-conditioning, sludge flocs broke up by the aquatic worm predation. The absolute zeta potential decreased to -8.27 mV, and the particle size increased from 36.64 μm to 48.05 μm. Proteins, polysaccharides and other organic substances in sludge EPS and microbial cells were released, with the viscosity reduced to 1.16 mPa·s and the bound water converted into free water. Besides, the number and abundance of representative filamentous Chloroflexi decreased, resulting in the enhancement of sludge dewatering performance. Overall, the aquatic worm conditioning process can be divided into two steps: Sludge destruction by the aquatic worm predation and sludge re-coagulation by filamentous bacteria as a skeleton.

A comparison of oxidation and re-flocculation behaviors of Fe2+/PAA and Fe2+/H2O2 treatments for enhancing sludge dewatering: A mechanism study

In this study, Fe²⁺ activated-PAA was developed as a novel technology to enhance sludge dewatering. The result showed that the filterability (CST₀/CST) enhanced by 4.20 ± 0.14 times more than the control, and the SRF and bound water content decreased from 4.58 ± 0.07 × 10¹³ m/kg and 2.11 ± 0.28 g/g dry sludge to 9.47 ± 0.05 × 10¹² m/kg and 1.27 ± 0.18 g/g dry sludge, respectively after the sludge was conditioned by 1.20 mM/g VSS Fe²⁺ and 1.20 mM/g VSS PAA. The dewatering performance, physicochemical properties, aggregation behaviors, and EPS fractions of sludge were compared before and after Fe²⁺/PAA and Fe²⁺/H₂O₂ conditionings. The results showed that Fe²⁺/PAA treatment was more competitive in enhancing dewaterability under neutral and alkaline conditions than Fe²⁺/H₂O₂ treatment but slightly weaker under acid conditions. Besides, it was found that the oxidation and re-flocculation behaviors were different in those two enhanced dewatering technologies due to the difference in the generated ROS. R-O was the primary radical in the Fe²⁺/PAA system, while OH was the major one in the Fe²⁺/H₂O₂ system. The mechanism analysis found that the Fe²⁺/PAA process caused harsher disintegration of sludge flocs, meaning more generation of fine particles. However, it exhibited less effect on reducing the energy barrier between sludge particles. Therefore, the Fe²⁺/PAA treated sludge presented weaker aggregation behaviors. The weaker aggregation was unfavorable for sludge dewatering because the weaker aggregated flocs were more easily fragmented, which hampered the consolidation of sludge cakes and removal of bound water. Moreover, loosely-bound extracellular polymeric substances, particularly tightly-bound extracellular polymeric substances, governed the sludge dewaterability.

Dewatering municipal wastewater sludge using electro-coagulation combined with added free nitrous acid

An electro-coagulation (EC) process combined with added free nitrous acid (FNA) improves sludge dewaterability. Under optimal conditions(EC voltage of 25 V, EC process time of 60 min, FNA dosage of 1.13 mg/L, pH value of 4.5), specific resistance to filtration (SRF) and water content (WC) was decreased by 89.57%, and 18.90%respectively. The EC process disrupted the sludge structure, reducing sludge particles' size (D50) from 59.5 to 50.5 μm. After adding FNA, the sludge cells lysed, and the DNA concentrations and soluble chemical oxygen demand (SCOD) increased from 6.07 μg/ml and 29 mg/L to 364 μg/ml and 588 mg/L, respectively. The conversion of Fe(II) to Fe(III) was enhanced. The addition of FNA after EC further improved the sludge dewaterability. Combined conditioning using EC and FNA can effectively destroy tightly bound extracellular polymeric substances (TB-EPS) and release bound water. In addition, the pH value is kept low, which benefits sludge dewaterability and the removal of heavy metals. The concentrations of Zn and Mn in the sludge cake were reduced by 92.3% and 69.0%, respectively. The Bureau of Reference (BCR) sequential extraction method showed increases in the percentages of the residual fractions of Zn and Mn, showing that EC combined with FNA is an efficient and versatile means of sludge conditioning.

A novel pre-magnetized ZVI/PS pretreatment for improving sludge dewaterability: The role of EPS fractions

The dewaterability of waste-activated sludge (WAS) has been extensively examined using zero-valent iron (ZVI)-based advanced oxidation processes (AOPs). However, the high dosage and low utilization efficiencies of ZVI cast doubt on the dependability and viability of ZVI-based AOPs. In this study, we successfully demonstrated pre-magnetization as an efficient, chemical-free, and ecological method for improving the efficiency of sludge dewatering by ZVI/persulfate (PS) process, in which the reduction ratios of capillary suction time (CST) and specific resistance to filtration (SRF) increased by 8.67% and 11.06% under optimal conditions, respectively. The highly active Fe²⁺ released during ZVI corrosion may be more essential than ZVI itself during PS activation, which could be strengthened by pre-magnetization. Both homogeneous and heterogeneous Fe²⁺ could react with PS to produce aqueous hydroxyl radicals (∙OH) and sulfate radicals (SO₄^-∙) as well as surface-bound ∙OH and SO₄^-∙, further decomposing bound-extracellular polymeric substances fractions, broking hydrophilic functional groups and compounds, altering protein secondary structure to expose more hydrophobic sites, and releasing abundant EPS-bound water. Due to the protection of tightly-bound extracellular polymeric substances (TB-EPS) and the competitive oxidation of organics released during the early disintegration stage, radical oxidation primarily occurs at extracellular levels, releasing a bit of intracellular water. Besides, polysaccharides in TB-EPS may function a more significant role in flocculation than proteins, and a porous structure favorable to drainage will be formed after the pre-magnetized ZVI/PS treatment. The cost-benefit analysis further reveals that the Pre-ZVI/PS process presents high reusability and utilization, making it potential for particle application in sludge dewatering.

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.

A novel conditioning approach for amelioration of sludge dewaterability using activated carbon strengthening electrochemical oxidation and realized mechanism

Sludge dewatering is an essential process for reduction of sludge volume to decrease cost of ultimate disposal. In this study, a novel method using activated carbon (AC) strengthening electrochemical (EC) treatment (EC/AC) was adopted to improve greatly sludge dewaterability. It was shown that capillary suction time (CST) and water content of dewatered sludge cake (Wc) were reduced to 55.9 ± 1.24 s and 64.3 ± 1.23%, respectively, under the optimal conditions of EC voltage 20 V, EC time 30 min and 0.2 g/g dry solid (DS) AC. AC with rich functional groups as "the third electrode" intensified electrooxidation by forming multiple microelectrodes and electron transfer capacity and conductivity of sludge were strengthened by AC in EC system, which were illustrated by electrochemical analysis. It could be found that zeta potential and particle size were increased and surface roughness was reduced after EC/AC treatment intensifying sludge hydrophobicity. Form the results of rheological behaviors of sludge, flowability was strengthened and viscosity was weakened under the conditioning of EC/AC. Besides, colloidal force and gel-like network strength were lessened, which was also verified by organic matters and percentage of inviable cells. At the same time, intracellular matters were released and degraded and bound water was released converting into free water. In addition, sludge compressibility and structural strength were increased and porous structure was formed facilitating water outflow via addition of mesoporous AC as skeleton builder, which eventually led to an improved separation efficiency of solid-water and sludge dewaterability. The results of heavy metals suggested that sludge cake after EC/AC treatment was favorable for land application.

Changes of network structure and water distribution in sludge with the stratified extraction of extracellular polymeric substances

The water in sludge is trapped within the extracellular polymeric substance (EPS) with gelatinous structure, greatly challenging the sludge deep dewatering. In this paper, the effect of the EPS viscoelasticity and the structural characteristics of sludge flocs on water distribution was revealed to provide a highly efficient approach in research on sludge dewatering. After biological, and physical method conditioning, the change of viscoelasticity and sludge network structure before/after EPS extraction was comprehensively explored, together with the sludge dewaterability and water distribution. The results suggested the proportion of capillary water and adsorption water carried in soluble EPS (S-EPS) was 59.17% and 40.83%, and that in tightly bound EPS (TB-EPS) was 54.77% and 45.23%, respectively. By contrast, the capillary water in loosely bound EPS (LB-EPS) accounted for as high as 99.99%. In comparison with raw sludge, adsorption water proportion in TB-EPS and S-EPS was reduced after lysozyme (LZM) or freezing-thaw conditioning, which was ascribed to reduction of EPS viscosity and the weakness of water adsorption capacity. Additionally, the sludge yield stress (τ_y) value first reduced and then increased with the extraction of EPS. Meanwhile, the consistency coefficient (k) also decreased from 4.23 Pa·sⁿ to 0.006 Pa·sⁿ and then slightly increased after LZM conditioning. This observation indicated the sludge system became sensitive to shearing, and its network structural strength as well as colloid elasticity first weakened and then slightly strengthened. In addition, after LZM or freezing-thaw conditioning, the sludge particle size significantly increased after TB-EPS extraction, while the sludge particle more easily absorbed water molecules, thereby increasing adsorption water and capillary water within the sludge flocs. This phenomenon also resulted in an increasing trend of capillary suction time (CST) after TB-EPS extraction, indicating the deterioration of sludge filtration performance.

Iron-based advanced oxidation processes for enhancing sludge dewaterability: State of the art, challenges, and sludge reuse

The increasing amount of sewage sludge produced in wastewater treatment plants (WWTPs) poses a great challenge to both environment and economy globally. As a requisite process during sludge treatment, sludge dewatering can significantly minimize the sludge volume and lower the operational cost for downstream transportation and disposal. Iron-based advanced oxidation process (AOP), a robust and cost-effective technique with relatively low technical barriers for high-level sludge dewatering, has been widely explored in the past 20 years. The development was mainly driven by the demands of efficient and sustainable sludge conditioning technology and the flexible sludge management approaches. The application of iron-based AOPs in sludge dewatering process attracts more and more attention. In this work, we discussed the current application of iron-based AOPs technology in the sludge dewatering processes in a holistic manner, summarized the factors affecting the sludge dewaterability in the treatment processes, and analyzed the mechanisms of iron-based AOPs to improve dewatering processes. Furthermore, we elaborated potential advantages, limitations, and challenges associated with implementing iron-based AOPs in the full-scale plants and shared the opportunities for sludge reutilization. This review aims to contribute to the development of highly efficient iron-based AOPs for sludge dewatering and offer perspectives and directions towards the new-generation of WWTPs with the sustainable and eco-friendly benefits.

Influence of persulfate on transformation of phosphorus and heavy metals for improving sewage sludge dewaterability by hydrothermal treatment

Activated persulfate oxidation has been proven to be an efficient advanced sludge treatment technique to improve sludge dewaterability. This study investigates the influence of persulfate on the transformation of phosphorus (P) and heavy metals (HMs) during the hydrothermal treatment of sewage sludge. The hydrothermal temperature, time, and persulfate concentration are optimized by a Box-Behnken design to obtain the best sludge dewaterability, which is expressed by capillary suction time (CST). The highest CST reduction efficiency is 90.5% at the optimal hydrothermal temperature, time, and concentration of persulfate, which are 145 °C, 2 h, and 150 mg/g dry sludge (DS), respectively. The distribution and transformation of P and HMs with different persulfate concentrations (100-200 mg/g DS) during the hydrothermal process are investigated. Results show that more than 90% of the P and HMs in the sludge are retained in sludge cakes after the hydrothermal treatment. The addition of SPS can make the P in the sludge cakes transform into more stable P species according to the extraction capacity of sequential extracts. It can be found from the ecological risk indexes of the HMs that the addition of SPS during the hydrothermal treatment of sludge can reduce the environmental risk of HMs. This study provides insights into the P and HM distribution and transformation during hydrothermal treatment with persulfate, providing a reference for sludge recovery strategies.

Enhanced waste activated sludge dewaterability by the ozone-peroxymonosulfate oxidation process: Performance, sludge characteristics, and implication

Dewatering treatment is an essential step to diminish sludge volume, cut down transportation costs, and improve subsequent disposal efficiency. In this study, ozone-peroxymonosulfate (O₃/PMS) oxidation process was employed to ameliorate sludge dewaterability. Sludge capillary suction time (CST) and water content (Wc) of dewatered sludge cake could reduce from 70.5 s and 81.93% to 26.7 s and 65.65%, respectively, under the optimal dosage of 30 mg/g TS O₃ and 0.4 mmol/g TS PMS. The increased sludge zeta potential, particle size, and fluidity promoted sludge dewatering performance apparently. The decreased hydrophilic, fluorescent EPS components and proteins/peptides-like + Lipids percentage in EPS as well as the ratio of α-helix/(β-sheet + random coil) of treated EPS protein secondary structure was greatly responsible for the enhanced sludge dewaterability. SO₄^- and OH were detected in ozone-peroxymonosulfate process to crack sludge flocs, eliminate hydrophilic substances and liberate bound water. Moreover, the concentrations of both heavy metals and polycyclic aromatic hydrocarbons (PAHs) of sludge after O₃/PMS conditioning were decreased, and the stability and toxicity of heavy metals were also reduced, except Zn. In conclusion, this work offered a comprehensive insight based on ozone-peroxymonosulfate (O₃/PMS) advanced oxidation for improving the sludge dewaterability and environmental implication.

Effects of the metabolic uncoupler TCS on residual sludge treatment: Analyses of the microbial community and sludge dewaterability potential

Residual sludge is a by-product with a large volume and complex composition from wastewater treatment plants. It is significant to reduce sludge volume to decrease the negative effects of sludge on environmental pollution and needless land use. We investigated the effects of uncoupler 3, 3', 4', 5-tetrachlorosalicylanilide (TCS) on the properties of sludge. After adding 0.12 g TCS/g VSS with 24 h mixing, the sludge concentration and total ATP content decreased by 51.1% and 60.8%, respectively. At the same time, the microbial community also changed significantly, leading to the decrease of richness and diversity. Additionally, the secretion of extracellular polymeric substances (EPS) reduced approximately 43% under the addition of 0.12 g/g VSS compared with the control. The decrement of EPS may be explained by the decreased relative abundance of functional bacteria (i.e. Chloroflexi reduced about 60% and Nitrospirota reduced about 31%). Notably, the addition of TCS before coagulation conditioning (FeCl₃) promoted the adhesion of sludge flocs according to the theory of Extended Derjaguin Landau Verwey Overbee (XDLVO), leading to the increased hydrophobicity of the residual sludge. Therefore, energy uncoupling has the potential of improving sludge dewaterability.

Effect of long-term low concentrations of TiO2 nanoparticles on dewaterability of activated sludge and the relevant mechanism: the role of nanoparticle aging

Nanoparticles can undergo aging phenomena in sewage treatment systems, which alter their physical and chemical properties. However, the effect of aged nanoparticles on the dewatering performance of activated sludge under long-term low concentrations is yet to be reported in sewage treatment systems. Here, we compared the chronic effects of pristine and aged TiO₂ nanoparticles on the sludge dewatering index, which includes specific resistance to filtration (SRF) and bound water (BW) in a sequencing batch reactor (SBR) at μg/L concentration levels, and analyzed the relevant mechanisms. The results indicated that aging in the sludge supernatant altered the photosensitivity and water stability of nanoparticles, which was mainly due to the changes in the zeta potential and energy band of the particle and was ultimately attributed to the combined effect of particle surface inclusions such as organic matter and inorganic salt. At 10 μg/L, nanoparticles reduced the sludge dewaterability, which observed an improvement at 100 μg/L. This is because 10 μg/L promoted the secretion of extracellular polymeric substances (EPS), which regulated the structure of sludge flora and increased the abundance of secreted quorum sensing-acyl-homoserine lactones (QS-AHL) and EPS genera, while the corresponding exposure results for 100 μg/L were the opposite, owing to the damage and necrosis effects caused by exposure under long-term light, which reduced EPS production and increased sludge density. Interestingly, aging could alleviate the effects of two exposure concentrations on sludge dewatering, mainly because of the decrease in the photoactivity of the nanoparticles. The results of this study show that environmental aging could delay, but not reverse the results of exposure to specific concentrations of nanoparticles. However, the significantly different ecological effects of photosensitive nanoparticles with two environmentally relevant concentration should be refined and confirmed again in freshwater environments to provide a basis for subsequent scientific management and control of photosensitive nanoparticles.

Fe(II)-activated sodium percarbonate for improving sludge dewaterability: Experimental and theoretical investigation combined with the evaluation of subsequent utilization

Fe(II)-activated sodium percarbonate (SPC) was an emerging technology for enhancing the dewaterability of waste activated sludge, and its operational parameters were systematically explored. The results showed that after the treatment by 1.20 mmol/g VSS SPC and 1.44 mmol/g VSS Fe(II) at initial pH 3.0, the water content and specific resistance to filtration remained at 76.05 ± 0.36% and 2.57 ± 0.08 × 10¹² m·kg^-1, respectively. The acid condition was instrumental in sludge dewatering, whereas overdosing Fe(II) or SPC imposed adverse effect. The conversion of EPS fractions was examined to elucidate the underlying mechanism, which indicated that a coexisting oxidation/flocculation process accounted for the improvement of sludge dewaterability. The stronger oxidative ·OH degraded the hydrophilic compounds (proteins and carbohydrates) of tightly-bound extracellular polymeric substance and the dissolved multivalence iron promoted solid-liquid separation. Additionally, the theoretical analysis (DFT calculation) demonstrated that the oxygen- and nitrogen-containing groups of EPS resulted in high-water holding capacity of sludge. The difficulty of destroying hydrophilic functional groups followed C=O > C-N > C-O during oxidation process. Moreover, Fe(II)/SPC treatment performed well in coliforms inactivation and phytotoxicity reduction compared with different ·OH-based advanced oxidation processes for sludge conditioning.

Deciphering the impacts of composition of extracellular polymeric substances on sludge dewaterability: An often overlooked role of amino acids

An investigation was conducted for waste activated sludge pretreated by different methods (e.g., ultrasonic, thermal, ozone, and acid/alkaline) in order to establish correlations between amino acids and parameters related to sludge dewaterability (e.g., capillary suction time (CST), specific resistance to filtration (SRF), proteins (PN) and polysaccharides (PS) in different fractions of extracellular polymeric substances (EPS), zeta potential, and particle sizes). The results indicated that glycine, serine, and threonine were the key identified amino acids correlated with parameters related to sludge dewaterability. To be exemplified, glycine showed positive correlations with the normalized CST (regression coefficient (R) = 0.72, p < 0.05), the normalized SRF (R = 0.74, p < 0.05), PN in soluble EPS (R = 0.89, p < 0.05), PS in soluble EPS (R = 0.56, p < 0.05), tryptophan-like PN in soluble EPS (R = 0.60, p < 0.05), and tryptophan-like PN in loosely-bound EPS (R = 0.58, p < 0.05). After adding extra glycine, serine, and threonine into sludge samples, sludge dewaterability was deteriorated. The hydrophilic functional groups of CO and C-OH were found to be more predominant in sludge with the presence of these amino acids. The Lewis acid-base interaction predominated in determining the net attraction among sludge flocs. Moreover, the presence of glycine, serine, and threonine resulted in high repulsive hydrophilic interaction, which deteriorated sludge dewaterability. This study emphasized the importance of amino acids in sludge dewatering and amino acids might be incorporated into parameters reflecting sludge dewaterability.

Identifying the key sludge properties characteristics in Fe2+-activated persulfate conditioning for dewaterability amelioration and engineering implementation

Fe²⁺-activated persulfate process has been introduced into sludge conditioning currently, however the key sludge properties characteristics are worthwhile comprehensively considering for the engineering implementation and management. The results indicated that both the optimal dosages of persulfate and Fe²⁺ were 0.6 mmol/gTS for sludge dewaterability amelioration, and the reduction efficiencies of capillary suction time (CST), specific resistance of filtration (SRF), and water content (Wc) of dewatered sludge cake reached to 90.5%, 97.2%, and 22.4%, respectively. Significantly, the persulfate and Fe²⁺ exerted distinctive roles in the conditioning process. The increased persulfate could promote the oxidatively disintegrated effect on sludge flocs, rendering the decrease of particle size. With the oxidative decomposition of the negatively charged biopolymers, sludge zeta potential rose gradually. However, Fe²⁺ contributed to more persulfate activation to generate free radicals, and the produced Fe³⁺ could further electrically neutralize the broken sludge fragments. The core mechanism of Fe²⁺-activated persulfate conditioning is "destroying and re-building" of sludge flocs. Noteworthily, EPS protein was oxidatively degraded more preferentially than EPS polysaccharide, and the decrease of the α-helix content of EPS protein was conducive to the enhancement of sludge dewaterability. Furthermore, the hydrophilic functional groups reduced clearly and element chemical states on sludge flocs altered pronouncedly, also the destroyed structure and microchannel facilitated the flowability of water. These findings provide theoretical and technical support for the practical engineering implementation of the Fe²⁺-activated persulfate conditioning process.

Performance and stability of biogas recirculation-driven anaerobic digestion system coupling with alkali addition strategy for sewage sludge treatment

Wastewater treatment plants are particularly challenging with the treatment and disposal of sewage sludge produced from the treatment units due to its high costs and environmental hazards. In this study, a biogas recirculation-driven anaerobic digestion (AD) system was developed with upward shear force being provided by biogas recirculation coupled with the alkali addition strategy, targeting biogas upgrading, sludge stabilization, and sludge flocculation simultaneously, thus reducing the sludge management costs. Compared to the conventional AD system, the novel biogas recirculation-driven AD system could achieve biogas upgrading with 10% higher CH₄ content. Besides, the combination of NaOH and Ca(OH)₂ addition strategy obviously improved sludge settleability and dewaterability compared to the single NaOH addition strategy. Owing to the attraction between negatively charged sludge particles and Ca²⁺ ions, the available Ca²⁺ in the former AD system may facilitate the re-flocculation and P immobilization in solid digestate, fix partial CO₂ with less CO₂ emission, and bridge with some sludge flocs. Moreover, 12.6% lower net cost for sludge management was achieved by this biogas recirculation-driven AD system together with the combination alkali addition strategy, which is regarded as a promising integrated multi-purpose system for sludge treatment.

A sodium dichloroisocyanurate-based conditioning process for the improvement of sludge dewaterability and mechanism studies

Sludge dewatering is necessary to reduce the volume of sludge for cost-effective transport and ultimate disposal. In this study, a novel combined chemical conditioning process was proposed to improve sludge dewatering performance in which sludge flocs were destructed by sodium dichloroisocyanurate (DCCNa) and re-flocculated by Al₂(SO₄)₃ and the mechanism was elucidated. The results showed that sludge capillary suction time (CST) dropped to 15.4 s and moisture content of dewatered sludge cake (Mc) deceased to 71.01% respectively, after the application of combined conditioning with the optimal dosage of 200 mg DCCNa/g dry solids (DS) and 80 mg Al₂(SO₄)₃/g DS. With chemical conditioning, sludge physicochemical properties were greatly changed. With the DCCNa application, the percentage of low-molecular-weight substances in soluble extracellular polymeric substances (S-EPS) increased. Also, the sludge zeta potential dropped from -16.85 mV to -25.45 mV and the median particle size (D50) decreased from 54.1 μm to 51.6 μm. However, the subsequent conditioning by Al₂(SO₄)₃ dosing not only led to an increment of 18% in the portion of macromolecules in S-EPS, but also increased the zeta potential and D50 to -10.74 mV and 53.2 μm, respectively. The bound water content in sludge declined from 2.92 g/g DS to 1.98 g/g DS after combined conditioning. We concluded that DCCNa disintegrated the sludge flocs and microbial cells leading to the release of bound water, fine particles and organic substances with negative charge, and the fine colloidal particles can be flocculated into large dense aggregations with the dosing of Al₂(SO₄)₃. In summary, the proposed combined conditioning provided a highly effective and environmental friendly approach to improve the sludge dewatering performance.

Surfactant-assisted thermal hydrolysis off waste activated sludge for improved dewaterability, organic release, and volatile fatty acid production

The surfactant-assisted thermal hydrolysis pretreatment (THP) of waste activated sludge (WAS) was investigated, focusing on the effect of the surfactant addition on the results of sludge disintegration, dewaterability, organic release, and production of volatile fatty acids (VFAs) via fermentation. Typical anionic surfactant sodium dodecyl sulfate (SDS) and cationic surfactant cetyl trimethyl ammonium bromide (CTAB) were used for the THP experiments. The supernatant of the THP-treated sludge was anaerobically fermented to determine its potential VFAs yield. The results showed that the surfactant addition, particularly CTAB, enhanced the hydrolysis and organic solubilization of the sludge during THP. CTAB addition led to a 36% increase of dissolved organic and a 27% increase of VFAs production. For the THP-treated sludge with the surfactant addition, its dewaterability was also greatly improved. When the CTAB dosage increased from 0 to 0.10 g/g VSS, the minimum capillary suction time (CST) of the sludge decreased from 205 to 50 s/g TSS, and the sludge particles became smaller and less negative with the zeta potential changing from -12.4 to -8.2 mV. Analysis of extracellular polymeric substances (EPS) of the sludge revealed that the surfactant addition increased the sludge disintegration and organic dissolution during the THP process. The surfactant-assisted THP is shown to be a promising technology to enhance the WAS treatment for improved sludge dewaterability, waste reduction, and resource recovery.

Comprehensive comparison of acidic and alkaline anaerobic fermentations of waste activated sludge

This study conducted a comprehensive comparison of acidic (R_5.0) and alkaline (R_10.0) anaerobic fermentations of waste activated sludge (WAS). The results showed that alkaline fermentation was able to increase biopolymer release and benefitted the production of volatile fatty acids (VFAs). However, large amounts of the released organic matter in the R_10.0 fermented liquid had low biodegradability unsuitable for the biological nutrient removal (BNR) process, resulting in increased C, nitrogen, and phosphorus loads in BNR effluent. Further, Al was more readily released than other metals and its maximum concentration reached 134.52 mg/L in R_10.0, 2.99 times higher than in R_5.0. The fermented sludge filterability was severely deteriorated at R_10.0, as indicated by the normalized capillary suction time and specific resistance to filtration. Considering these findings, VFAs from WAS via acidic fermentation may represent a suitable carbon source for direct use in the BNR process.

Coupling hydrothermal carbonization and anaerobic digestion for sewage digestate management: Influence of hydrothermal treatment time on dewaterability and bio-methane production

Hydrothermal carbonization (HTC) technology is addressed in the framework of sewage digestate management. HTC converts digestate into a stabilized and sterilized solid (the hydrochar) and a liquor (HTCL) rich in organic carbon. This study aims to optimize the HTC operating parameters, namely the treatment time, in terms of hydrochar production, HTC slurry dewaterability, HTCL bio-methane yields in anaerobic digestion (AD), and process energy consumption. Digestate slurry was processed through HTC at different treatment times (0.5, 1, 2 and 3 h) at 190 °C, and the dewaterability of the treated slurries was addressed through capillary suction time and centrifuge lab-testing. In addition, biochemical methane potential (BMP) tests were conducted for HTCL under mesophilic conditions. Results show that by increasing the HTC treatment time the dewaterability was further improved, ammonium concentration in HTCL increased, and methane potential of HTCL decreased. 0.5 h HTCL had the highest bio-methane potential of 142 ± 3 mL CH₄/g COD yet the treatment time was not sufficient for improving the slurry's dewaterability. HTC treatment time of 1 h at 190 °C was identified as the optimum trade-off for improved dewaterability and utilisation of HTCL for biogas production. 1 h HTCL bio-methane potential can cover around 25% of the HTC and AD thermal and electrical energy needs without considering the eventual use of the hydrochar as a biofuel.

Study of sludge conditioning using organic acids chelated ferrous ion catalyzed NaClO oxidation: Evolution of extracellular polymeric substances and floc structure

In previous studies, we interestingly found that several ligands (e.g., nitrilotriacetate and humic acid) could significantly accelerate the oxidation of NaClO by ferrous ion, via complexing with Fe(II). This inspired us to hypothesis that organic acids (OA) chelated Fe(II) might enhance the NaClO oxidation for improving sludge dewaterability. In this work, the OA chelated ferrous ions activated NaClO (OA-Fe(II)-NaClO) process was utilized for sludge conditioning, which exhibited significantly improved dewatering performance, as evidenced by capillary suction time (CST), specific resistance to filtration (SRF) and cake moisture of sludge. The normalized CST (CSTn) and SRF decreased to 1.22 s × L/g and 3.24 × 10⁸ m/kg, when ratio dosage of [ClO^-]/[Fe²⁺] (NaClO: 0.75%(v/v)) was 1 at pH of 2. During the NaClO activation by Fe(II), tightly bound extracellular polymeric substances (TB-EPS) was cracked into soluble EPS (S-EPS), followed by oxidizing into smaller molecular organic matters due to the production of hydroxyl radicals. Meanwhile, OA complexed to ferrous ion inhibited hydrolysis of ferric ions, leading to better catalytic performance of NaClO under neutral pH conditions. In addition, the molecular structure of OA determined the low complexing ability and steric hindrance for OA-Fe(II). When molar ratio of [Oxalic acid]/[Fe²⁺] was 1, oxalic acid-Fe(II)-NaClO improved sludge dewaterability significantly due to the coagulation and skeleton builder properties of formed iron oxalate precipitates. This work provides an eco-friendly and cost-effective method for improving sludge dewaterability.

Dewaterability of sewage sludge conditioned with a graft cationic starch-based flocculant: Role of structural characteristics of flocculant

Coagulation/flocculation is one of the most extensive and cost-effective pretreatments to improve the dewaterability of sludge in water treatment plants. In this study, three series of graft cationic starch (St)-based flocculants with distinct structural characteristics including charge density (CD), graft-chain length (L), and graft-chain distribution (N) were synthesized by graft copolymerization of [(2-methacryloyloxy-ethyl) trimethyl ammonium chloride] and acrylamide onto St backbone. The structural effects of these St-based flocculants on the sludge dewaterability have been quantitatively analyzed by using a second-order polynomial model according to phenomenological theory. The predicted dewatering performance and optimal dose were fully consistent with the experimental results. On the basis of this established model, the dewatering mechanisms were discussed in detail by combination of the analysis of the changes in filter cake moisture content, specific resistance of filtration, bound water content, compression coefficient, extracellular polymeric substances fractions and components, spatial distributions of proteins and polysaccharides, microstructures of sludge cake, and flocs properties in the dewatering process. This graft St-based flocculant, with the structural characteristics of high CD, long L, and low N, exhibited superior sludge dewaterability because of the enhanced charge neutralization and bridging flocculation effects. Among these three structural factors, CD played a more important role in improvement of sludge dewaterability than L and N due to the dominant effect of charge neutralization. This study provided a better understanding of structure-activity relationship of these grafting modified flocculants, which was of significant guidance for the exploit and design of novel and efficient flocculants for improvement of sludge dewaterability.

Evaluation of the combined effect of sodium persulfate and thermal hydrolysis on sludge dewatering performance

This innovative study makes use of a thermal hydrolysis process (THP) and the conditioner sodium persulfate (SPS) to improve the dewaterability of sewage sludge. The best-operating conditions were optimized using response surface methodology (RSM): 100 mg/g of dry solids (DS) of SPS, 101 min of reaction time of THP, and a temperature of 200 °C. Distribution of extracellular polymeric substances (EPS), zeta potential, bound water, and solid characters were analyzed to reveal the mechanisms involved in the dewatering process. These results indicate that the sewage sludge after treatment (SPS combined with THP) had a superior dewaterability. The specific resistance to filtration (SRF) under the best conditions was 0.51 × 10¹¹ m/kg, decreasing by 91.65% compared to the raw sludge (RS) (6.11 × 10¹¹ m/kg). This mechanism could be explained as follows: (1) Aromaticity and hydrophobicity of sludge cake after SPS + THP treatment was increased; (2) sludge flocs were re-flocculated by charge neutralization, giving rise to a loose and porous structure; (3) the structure of extracellular polymeric substances and cells was destroyed, and the bound water was released. Overall, the conditioning by combination of SPS and THP is an effective mean to improve sewage sludge dewaterability. Graphical abstract.

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.

New insights into the effect of extracellular polymeric substance on the sludge dewaterability based on interaction energy and viscoelastic acoustic response analysis

To elucidate the effects of extracellular polymeric substance (EPS) on the sludge dewaterability, this study comparatively investigated the changes in EPS composition and spatial distribution, together with the sludge dewaterability after lysozyme (LZM) conditioning. The protein concentration in the tightly bound EPS (TB-EPS) increased from 3.47 mg g^-1 DS to 4.99 mg g^-1 DS within the first 2 min, then gradually decreased, which could be described by a piecewise linear function. Unlike TB-EPS, the protein content variation trend in both soluble EPS (S-EPS) and loosely bound EPS (LB-EPS) followed the typical first-order kinetics. Additionally, the extended DLVO theory was employed in combination with viscoelastic acoustic response analysis to further explore the impact of EPS composition on water adhesion and microbial cell. After the extraction of S-EPS from the conditioned sludge, the adsorption free energy (ΔG_adh) of EPS ascended to -61.05 mJ m^-2, indicating the weakened microbial hydrophobicity. By contrast, the ΔG_adh value declined after the subsequent extraction of LB-EPS and TB-EPS. Meanwhile, the adsorption potential energy between S-EPS and microbial cells showed an increasing trend, whereas the repulsion potential energy between TB-EPS and microbial cells fell to 1.40 × 10⁴ kT, signifying a weakened adsorption capacity to water. Accordingly, the viscosity and shear modulus of each EPS layer were reduced after conditioning, which contributed to the transformation of bound water into free water. These changes reasonably explained the results that the water content in the dewatered sludge after conditioning was reduced to 58.54%, and the bound water content decreased by 15.06%.

Coupling biogas recirculation with FeCl3 addition in anaerobic digestion system for simultaneous biogas upgrading, phosphorus conservation and sludge conditioning

The high costs involved in sewage sludge treatment and disposal in wastewater treatment plants (WWTPs) not only bring about improper sludge disposal and thus environmental pollutions, but also limit the investment on construction of WWTPs, especially in rural areas or low-income regions. This comparative study examined the effect of biogas recirculation coupled with chemical addition in a semi-continuous anaerobic digester for sludge treatment, which was proven to achieve biogas upgrading, phosphorus conservation and sludge conditioning simultaneously, largely reducing the sludge treatment cost. Results show that FeCl₃ addition coupling biogas recirculation can improve sludge dewaterability by 94% in comparison to 75% by equivalent MgCl₂ addition, and 97% phosphorus in digestate can be conserved in solid with formation of vivianite-like crystals. Biogas recirculation can enhance CH₄ yield and content by 13% and 11%, respectively, likely attributable to the increased relative abundances of both hydrogenotrophic Methanomicrobiales and acetoclastic Methanosarcinales.

Role of extracellular polymeric substances in sludge dewatering under modified corn-core powder and sludge-based biochar pretreatments

Extracellular polymeric substances (EPS) which wrapped on sludge particles were deemed to hinder the outflowing of combined water in sludge system. The complex composition of EPS was the bottleneck for revealing its relationship with sludge dewaterability. In this study, a combined modified corn-core powder (MCCP) and sludge-based biochar (SBB) condition was executed to treat sludge for enhancing dehydration performance, and the concentration and the form distribution of organics in EPS, the variances of protein secondary structures were investigated. Correlation between the sludge dewaterability and EPS components were performed, found strong correlations among the net sludge solids yield (YN) and the specific resistance of filtration (SRF) (R = -0.923), Zeta potential (R = -0.971). Furthermore, the relationship between the secondary structures of protein and dehydration performance were strong related. With the optimal dosage of SBB and MCCP, aggregated strands and α -helix were released, indicated that the unfolding and despiralization in soluble EPS (S-EPS) were improved, disordered the sludge network, reduced the flowing resistance of bound water, finally enhancing sludge dewaterability.

A novel Fe2+/persulfate/tannic acid process with strengthened efficacy on enhancing waste activated sludge dewaterability and mechanism insight

As an essential section before final sludge disposal, sludge dewatering has currently been one of the focus issues. In this study, an innovative Fe²⁺/persulfate/tannic acid (TA) process was verified to further strengthen systemic efficacy on enhancing sludge dewaterability, compared with the conventional Fe²⁺/persulfate process. With the efficient TA/Fe²⁺ (molar ratio) of 0.25 added in Fe²⁺ (0.3 mmol/gTS (total solid))/persulfate (0.6 mmol/gTS) process, sludge dewaterability was enhanced remarkably. Capillary suction time, specific resistance to filtration, and water content of dewatered sludge cake were further reduced by 61.5%, 35.3%, and 6.4% than these in Fe²⁺/persulfate. Sludge supernatant viscosity was further reduced by 86.7% due to the more removal of extracellular polymeric substances (EPS). The secondary structure of EPS protein changed apparently and fluorescent components of EPS decreased distinctly. Sludge functional group contents were observed to be lower. TA effectually increased sludge particle size and heightened sludge flocculability, rendering the large and compact aggregations. Moreover, TA accelerated the recovery of Fe²⁺, facilitating persulfate activation to generate more SO₄^·- and ^·OH for EPS disruption and cell lysis in the conditioning system. These findings provided a novel approach based on the Fe²⁺/persulfate process in sludge treatment for desirable dewaterability.

High-level waste activated sludge dewaterability using Fenton-like process based on pretreated zero valent scrap iron as an in-situ cycle iron donator

A novel, recyclable, and rapid pre-ultrasound-thermal-acid-washed zero valent scrap iron/hydrogen peroxide (UTA-ZVSI/H2O2) method has been developed to effectively enhance waste activated sludge (WAS) dewaterability. The effects of UTA ultrasound densities, UTA temperature, newly generated iron solution, H2O2 concentrations, and WAS conditioning time on the WAS dewaterability were investigated using a bench-scale system. Results indicated that the UTA-ZVSI/H2O2 treatment significantly improved the WAS dewaterability. The water content of the dewatered cake decreased to 44.15 ± 0.98 wt% during optimal operational conditions, which was significantly lower than that achieved using Fenton-based processes. Based on this outcome, a three-step treatment mechanism involving UTA-ZVSI/H2O2 has been developed, including iron flocculation, hydroxyl radical oxidation, and skeleton building. The dewatering efficiencies of three types of representative WAS were consistently effective in the UTA-ZVSI/H2O2 reactor for up to 15 cycles. Efficiencies levels were significantly higher than those achieved with Fenton-based processes. Economic analysis illustrated that the developed UTA-ZVSI/H2O2 system was the most cost-effective among other WAS dewatering treatments. In addition, the treatment system significantly alleviated toxicity of heavy metals and phytotoxicity in the dewatered sludge. This supported subsequent agricultural use. In summary, this study provided a comprehensive and useful basis for improving WAS dewatering and subsequent disposal.

Profiling of amino acids and their interactions with proteinaceous compounds for sewage sludge dewatering by Fenton oxidation treatment

During advanced oxidation treatment for enhancing sludge dewaterability, the peptide chains of protein can be decomposed into amino acids. Protein exhibits a great impact on sewage sludge dewaterability. However, the role of amino acids in sludge dewatering remains unclear. In this study, among the 23 types of amino acids investigated, tryptophane (Trp) and lysine (Lys) were identified as the key amino acids affecting sludge dewaterability during Fenton oxidation treatment. The content of lysine showed positive correlations with capillary suction time (CST), specific resistance to filtration (SRF), and bound water content, and the concentrations of total protein, low molecular weight protein, amines and amides, and 3-turn helix of proteinaceous compounds in bound extracellular polymeric substances (EPS), while the content of tryptophane showed negative correlations with the above parameters. The amino acids may be sourced from damage of the membrane and ribosomal proteins by hydroxyl radicals, and the peptide bonds connected with tryptophane were more inclined to be decomposed than other amino acids. Particularly, more amino acids of tryptophane can result in more hydrophobic interaction, and less necessary energy barrier for aggregation of particles. As such, regulating protein degradation towards production of tryptophane may be related with enhanced sludge dewaterability by Fenton oxidation treatment.

Fe(II) catalyzing sodium percarbonate facilitates the dewaterability of waste activated sludge: Performance, mechanism, and implication

In this work, Fe(II) catalyzing sodium percarbonate (Fe(II)/SPC) was managed to facilitate waste activated sludge (WAS) dewatering for the first time. The results showed that after WAS was treated by 20 mg/g total suspended solids (TSS) Fe(II) and 50 mg/g TSS SPC, the water content of sludge cake (WC_SC) by press filtration and capillary suction time (CST) dropped from 90.8% ± 1.6% and 96.1 ± 4.0 s (the control) to 55.6% ± 1.4% and 30.1 ± 2.5 s, respectively. The mechanism investigations indicated that four intermediates or products (i.e., •OH, H₂O₂, Fe(II), and Fe(III)) generated in the Fe(II)/SPC process were responsible for the improved WAS dewaterability, and •OH and Fe(III) were the two major contributors. It was found that •OH collapsed and fragmented extracellular polymeric substances, damaged cell wall and permeabilized cytoplasmic membrane, and transformed conformation of the extracellular proteins secondary structure via both affecting the hydrogen bond maintaining α-helix and cracking disulfide bond in cysteine residues while Fe(III), the oxidization product of Fe(II), decreased the surface electronegativity and water-affinity surface areas of WAS flocs. As a result, the bound water release, flocculability, surface hydrophobicity, drain capability, and flowability of WAS flocs were strengthened whereas the compact surface structure, colloidal forces, network strength, gel-like structure, and apparent viscosity of WAS flocs were weakened. In addition, Fe(II)/SPC process also reduced the recalcitrant organics and fecal coliforms in sludge, which facilitated land application of dewatered sludge. The findings acquired in this work not only deepens our understanding of Fe(II)/SPC-involved WAS treatment process but also may guide engineers to develop both effective and promising strategies to better condition WAS for dewatering in the future.

Mechanism of zero valent iron and anaerobic mesophilic digestion combined with hydrogen peroxide pretreatment to enhance sludge dewaterability: Relationship between soluble EPS and rheological behavior

This study proposed a novel two-step conditioning strategy to enhance activated sludge (AS) dewatering performance. The method involved a zero valent iron (ZVI), anaerobic mesophilic digestion (AMD) process, and hydrogen peroxide (H₂O₂) oxidation. Response surface methodology (RSM) was applied to achieve optimum dewatering conditions. After the combined conditioning, dewatering was significantly better in the treated sludge compared to the raw AS. The specific resistance of filtration (SRF) of the treated sludge decreased to 2.48 × 10¹¹ m/kg; this SRF level was 93.60% lower compared to the raw AS. The bound water content (BWC) decreased to 1.19 g/g dry solid (DS); this BWC level was 15.2% lower compared to the raw AS. The water content of the treated sludge cake decreased to 44.18 ± 0.46%. An economic analysis shows that ZVI-AMD-H₂O₂ can be used in real-world settings. Investigations of the underlying mechanisms showed that small block structures were formed after conditioning; viscosity and the colloidal forces of the sludge decreased; and organic matter and BWC were released from inner extracellular polymeric substances (EPS) layers to form soluble (SB)-EPS. This study illuminated the relationship between SB-EPS and the rheological behavior of AS. There is a high correlation coefficient between rheological parameter τ_y and N-containing substances in SB-EPS (R = -0.993, p < 0.05). The ZVI-AMD-H₂O₂ process effectively changed the EPS content, especially protein materials. This led to a decrease in AS viscosity and an increase in sludge dewaterability.

Ultrasonic coupled bioleaching pretreatment for enhancing sewage sludge dewatering: Simultaneously mitigating antibiotic resistant genes and changing microbial communities

In this study, ultrasonic as a pretreatment coupled with bioleaching was used to enhance sludge dewaterability. Changes in microbial diversity and antibiotic resistant genes (ARGs) were studied during the combined treatment process. The results show that under optimal conditions, combined ultrasonic and bioleaching treatment led to decreases in the specific resistance of filtration and bioleaching time by 7.59% and 12.5%, respectively, compared with single bioleaching process. Using high pressure filtration system, the water content of sludge cake treated by the combined treatment was decreased to 58.04%, which was 10.04% lower than bioleaching sludge. After combined treatment, the microbial diversity and the total number of bacteria in the sludge decreased significantly, which caused the decreases in the absolute abundance of sulfonamide and tetracycline ARGs by 1.56-1.58 and 0.34-1.23 log units, respectively. However, the decrease in the total bacterial biomass was greater than the decrease in the number of potential hosts carrying the tetracycline ARG, resulting in an increase in the relative abundance of tetracycline gene. Furthermore, this study proposed a mechanism of the dewatering and ARGs, involving the combined ultrasonic and bioleaching treatment: Firstly, ultrasonic cavitation causes extracellular polymeric substances (EPS) to fall off the surface of sludge; Secondly, this faster and directly makes bacteria cells affected by bio-acidification and bio-oxidation. In this case, the cells could be more easily destroyed by the combined ultrasonic and bioleaching treatment, compared with individual bioleaching treatment; As a result, stronger dewaterability and more removal rates of ARGs were achieved under the combined treatment. The economic analyses showed that the combined ultrasonic and bioleaching treatment is a more practical and economical technique for achieving deep dewatering of sludge.

Municipal sludge dewatering properties and heavy metal distribution: Effects of surfactant and hydrothermal treatment

Extracellular polymeric substances (EPS) play an important role in reducing moisture content and removing heavy metals from municipal sludge. In this study, cetyl trimethyl ammonium bromide (CTAB) and primary alcohol ethoxylate (AEO₉) were used to prepare complex surfactants. Municipal sludge was treated with mixed surfactants and hydrothermal treatment (HTT) to test the sludge dewatering and heavy metal adsorption performance of EPS treated with varying proportions and contents of surfactants. The results showed that EPS with different proportions had different adsorption capacities for major heavy metals in sludge was 20.48%-29.82%. The adsorption capacity of EPS on heavy metals is significantly positively correlated with protein content (R² = 0.9266, P = .00). There is no significant correlation between polysaccharides (R² = 0.0092, P = .00) and nucleic acids (R² = 0.0638, P = .00) and heavy metal adsorption capacity. Combined treatment of 10% CTAB, 15% AEO₉, and HTT produced the minimal bound water content in treated sludge, zeta potential increased by 49.2 mV and sludge specific resistance is only 0.66% of the raw sludge.

Understanding the role of cellulose fiber on the dewaterability of simulated pulp and paper mill sludge

Pulp and paper mill sludge (PPMS) mainly consists of primary sludge and secondary sludge (activated sludge). Before the disposal and utilization of PPMS, it is dewatered due to high water content. Cellulose fiber is a key and characteristic component in PPMS. In this paper, the role of cellulose fiber on the dewaterability of simulated PPMS (the mixture of cellulose fibers and activated sludge) was investigated, including the fiber content, fiber length and fibrillation degree. The results showed that the specific resistance to filtration (SRF) of simulated PPMS decreased with the increase of fiber content (0-30 w%) and also decreased with the decrease of average fiber length (0.597-0.303 mm) noticeably. However, the SRF increased dramatically when the average fiber length reduced further to 0.189 mm. The fiber fibrillation had no obvious effect on the sludge SRF. Besides, the addition of cellulose fiber had little impact on the sludge compressibility. The role of cellulose fiber in the PPMS is to form aggregates with high fiber-sludge adhesion and large floc size, which relieves the blinding effect and changes the permeability of sludge cake respectively, resulting in improving sludge filterability.

Effect of extracellular polymeric substances (EPS) conditioned by combined lysozyme and cationic polyacrylamide on the dewatering performance of activated sludge

Extracellular polymeric substance (EPS) and the water within it account for about 80% of the total sludge mass, significantly correlated with sludge charge properties, particle size, and dewaterability, while their relation is still ambiguous. To clarify the effect of EPS characteristics on the activated sludge dewaterability, the sludge conditioned by cationic polyacrylamide (CPAM) and lysozyme alone or in combination was comparatively investigated by the means of the SPSS17 software incorporation with chemical analysis. According to the results, the combined conditioning increased both dewatering extent and dewatering rate with the water content as low as 57.79%. It was mainly attributed to the destruction of microbial cell wall and EPS structure by enzymatic conditioning, beneficial for the release of protein (PN) and polysaccharide (PS), as well as the conversion of intracellular water and some bound water into free water. Additionally, Pearson's correlation and factor analysis confirmed the significant influence of EPS properties on sludge dewaterability and explored their detailed relationship. It was indicated the most crucial factors consisted of PNS (PN in S-EPS), PNL (PN in LB-EPS), PNT (PN in TB-EPS), and PST (PS in TB-EPS) accounted for 72.83% in all of the total variance for the contribution to the dewatered water content. Moreover, the high concentration of PNS and PNL led to the zeta potential rising to -9.74 mV, and the destruction of EPS structure was favorable for sludge to form smaller particle size and compact floc structure. All the results were confirmed by the microstructure changes of the sludge flocs.