Anaerobically-digested sludge conditioning by activated peroxymonosulfate: Significance of EDTA chelated-Fe2

Enhancing volatile fatty acids production from waste activated sludge: The role of pretreatment by N,N-bis(carboxymethyl)-l-glutamate (GLDA)

N,N-bis(carboxymethyl)-l-glutamate (GLDA) is an eco-friendly chelating agent that effectively extracts multivalent metal ions from waste activated sludge (WAS) flocs, which could potentially alter their structure. However, the effect of GLDA on the production of volatile fatty acids (VFAs) from WAS is not well known. Here, we demonstrate that pretreatment with GLDA at a concentration of 200 mmol per kg VSS results in a significant increase of 142% in extractable extracellular polymeric substances and enhances the total VFAs yield by 64% compared to untreated samples. We reveal GLDA's capability to mobilize organic-binding multivalent metal ions within sludge flocs. Specifically, post-pretreatment analyses showed the release of 69.1 mg L-1 of Ca and 109.8 mg L-1 of Fe ions from the flocs, leading to a more relaxed floc structure and a reduced apparent activation energy (10.6 versus 20 kJ mol-1) for WAS solubilization. Molecular dynamic simulations further demonstrate GLDA's preferential binding to Fe3+ and Ca2+ over Mg2+. Our study suggests that GLDA pretreatment causes minimal disruption to reactor stability, thereby indicating the stability of microbial community composition. GLDA has emerged as a viable pretreatment agent for enhancing volatile fatty acids production from waste activated sludge.

Effect of microwaves combined with peracetic acid to improve the dewatering performance of residual sludge

The activated sludge process plays a crucial role in modern wastewater treatment plants. During the treatment of daily sewage, a large amount of residual sludge is generated, which, if improperly managed, can pose burdens on the environment and human health. Additionally, the highly hydrated colloidal structure of biopolymers limits the rate and degree of dewatering, making mechanical dewatering challenging. This study investigates the impact and mechanism of microwave irradiation (MW) in conjunction with peracetic acid (PAA) on the dewatering efficiency of sludge. Sludge dewatering effectiveness was assessed through capillary suction time (CST) and specific resistance to filtration (SRF). Examination of the impact of MW-PAA treatment on sludge dewatering performance involved assessing the levels of extracellular polymeric substances (EPS), employing three-dimensional excitation-emission matrix (3D-EEM), Fourier transform-infrared spectroscopy (FT-IR), and scanning electron microscopy. Findings reveal that optimal dewatering performance, with respective reductions of 91.22% for SRF and 84.22% for CST, was attained under the following conditions: microwave power of 600 W, reaction time of 120 s, and PAA dosage of 0.25 g/g MLSS. Additionally, alterations in both sludge EPS composition and floc morphology pre- and post-MW-PAA treatment underwent examination. The findings demonstrate that microwaves additionally boost the breakdown of PAA into •OH radicals, suggesting a synergistic effect upon combining MW-PAA treatment. These pertinent research findings offer insights into employing MW-PAA technology for residual sludge treatment.

Enhanced dewaterability and triclosan removal of waste activated sludge with iron-rich mineral-activated peroxymonosulfate

High water and pharmaceutical and care products (PPCPs) bounded in sludge flocs limit its utilization and disposal. The advanced oxidation process of perxymonosulfate (PMS) catalyzed by iron salts has been widely used in sludge conditioning. In this study, two iron-rich minerals pyrite and siderite were proposed to enhance sludge dewatering performance and remove the target contaminant of triclosan (TCS). The permanent release of Fe2+ in the activation of PMS made siderite more effective in enhancing sludge dewater with capillary suction time (CST) diminishing by 60.5 %, specific resistance to filtration (SRF) decreasing by 79.2 %, and bound water content (BWC) dropping from 37.1 % to 2.6 % at siderite/PMS dosages of 0.36/0.20 mmol/g-TSS after 20 min of pretreatment. Pyrite/PMS performed slightly inferior under the same conditions and the corresponding CST and SRF decreased by 51.5 % and 71.8 % while the BWC only declined to 17.8 %. Rheological characterization was employed to elucidate the changes in sludge dewatering performance, with siderite/PMS treated sludge showing a 48.3 % reduction in thixotropy, higher than 28.4 % of pyrite/PMS. Oscillation and creep tests further demonstrated the significantly weakened viscoelastic behavior of the sludge by siderite/PMS pretreatment. For TCS mineralization removal, siderite/PMS achieved a high removal efficiency of 43.9 %, in comparison with 39.9 % for pyrite/PMS. The reduction in the sludge solids phase contributed the most to the TCS removal. Free radical quenching assays and EPR spectroscopy showed that both siderite/PMS and pyrite/PMS produced SO4-·  and ·OH, with the latter acting as the major radicals. Besides, the dosage of free radicals generated from siderite/PMS exhibited a lower time-dependence, which also allowed it to outperform in destroying EPS matrix, neutralizing the negative Zeta potential of sludge flocs, and mineralizing macromolecular organic matter.

Quantitative probing of reactive oxygen species and their selective degradation on contaminants in peroxymonosulfate-based process enhanced by picolinic acid

The processes of Fe(III) activated peroxymonosulfate (PMS) in degrading contaminants have been extensively studied. Herein, a biodegradable chelating agent, picolinic acid (PICA), was introduced to the PMS/Fe(III) process to improve the reaction efficiency. The emphases of this study were placed on the quantification of steady-state concentrations of reactive oxygen species (ROS). Experiments presented that five types of ROS, including Fe(IV), SO4•-, HO•, 1O2 and O2•- coexisted in this system. Four typical probe compounds were used to quantify the steady-state concentration of ROS under different variables. The steady-state concentration of Fe(IV) ([Fe(IV)]ss) was 3-5 orders of magnitude higher than that of other ROS, followed by 1O2 and SO4•-, whereas HO• had the lowest concentration. The reaction between PMS and PICA was first explored in our study and results showed that 1O2 and O2•- would form in this reaction. Owing to the hybrid oxidation by multiple ROS, this system showed high oxidation capacity, and could effectively degrade a variety of pollutants. The contributions of ROS to the alleviation of pollutants varied depending on their concentrations and specific reactivity of substrates. Generally, organic contaminants with phenol structures were prone to react with Fe(IV). Overall, this study compared the steady-state concentrations of different ROS and revealed the intrinsic ROS formation mechanisms.

Insights into the role of ·OH generated in Fe2+/CaO2/coal slime system for efficient extracellular polymeric substances degradation to improve dewaterability of sewage sludge

The disposal of massive sewage sludge and coal slime is a problem facing municipalities in China. A hypothesis for the co-disposal of sludge and coal slime is proposed to improve dewaterability by utilizing the beneficial role of coal slime as a filter assist and CaO₂ enhanced system in this research. Results showed that capillary suction time, specific resistance to filtration and water content decreased dramatically from 49.3 s, 13.2 × 10¹² m/kg and 84.85% to 19.1 s, 1.0 × 10¹² m/kg and 50.07%, respectively, under the optimal conditions with 0.3/0.1/0.3-Fe²⁺/CaO₂/coal slime g/g DS. The hydroxyl radicals generated in the Fe²⁺/CaO₂ process acted on extracellular polymeric substances (EPS), resulting in a drop in the ratio of α-helix/(β-sheet + random coil) in the secondary structure of EPS proteins and a reduction in the concentration of aromatic proteins and tryptophan-like substances in TB-EPS, thereby enhancing the sludge dewaterability. Furthermore, coal slime as the skeleton building material induced a rise in sludge particle size and contact angle, lowering the hydrophilicity, compressibility of sludge and providing more channels to facilitate water flow. This work verified the promising application prospect of the Fe²⁺/CaO₂/coal slime combined system in the enhancement of sludge dewaterability.

Improving sludge dewaterability via Fe2+ chelated citrate activated peroxydisulfate oxidation

Citrate (Ct) was chosen as a typical chelator used in the Fe²⁺-peroxydisulfate (PDS) process to improve sludge dewaterability. The PDS-Fe²⁺-Ct process exhibited better performance in sludge dewatering than PDS-Fe²⁺. Specifically, with a PDS dosage of 1.2 mmol/g VS, the molar ratio of PDS/Fe²⁺ and Ct/Fe²⁺ were 4:5 and 1:4, respectively, the capillary suction time decreased from 155.8 to 24.8sec, and the sludge cake water content decreased from 82.62% to 64.11% (-0.06MPa). The oxidation led to a reduced negative charge and a decrease in particle size. The enhanced sludge dewaterability and changes of sludge properties were attributed to the decomposition of extracellular polymeric substances, and it was explored by protein, polysaccharide, 3D-EEMs, and FT-IR. Additionally, the quenching experiments of radical species demonstrated that SO₄^-• played a more important role than •OH, and its productivity was improved with the addition of Ct. Moreover, the reasons for the improved productivity of radicals with the addition of Ct were discussed. The results of this study could serve as a basis for improving sludge dewatering using the PDS-Fe²⁺-Ct process and suggest that the addition of Ct may improve the productivity of SO₄^-• in the activation of PDS via Fe²⁺.

Sludge disintegration and phosphorus migration in anaerobic fermentation of waste activated sludge by the addition of EDTA-2Na

The effects of the addition of EDTA-2Na on sludge disintegration and phosphorus (P) migration during anaerobic fermentation (AF) of waste activated sludge (WAS) are investigated. The efficiency of sludge disintegration was positively correlated with the dose of EDTA-2Na from 0.5-2.0 g/g SS, and an enormous quantity of P was liberated into the aqueous phase, accompanied by sludge disintegration. The proper dose of EDTA-2Na for P release from WAS was 1.5 g/g SS, with an orthophosphate concentration of 394.72 mg/L. P release was more consistent with the pseudo second-order kinetic model. The migration of P species during AF with EDTA-2Na addition was also studied. Orthophosphate was the main species in both of the liquid phase and the loosely bound extracellular polymeric substances (EPS), but organic P (OP) was much more abundant in tightly bound EPS. Inorganic P (IP) was the dominant P speciation in the solid and was mainly distributed in the fraction of non-apatite IP, which accounted for more than 62.8% of IP in the presence of EDTA-2Na. In addition, both IP and OP in the solid contributed to the accumulation of P and the former was outperformed. Furthermore, the increased total dissolved P mainly came from cells. However, the fermented sludge tended to be smaller and to have low compressibility, which is detrimental to its further treatment.

Iron-rich digestate biochar toward sustainable peroxymonosulfate activation for efficient anaerobic digestate dewaterability

In this study, a suite of Fe-rich biochars derived from Fenton-like treated digestate (Fe-BC) were fabricated under different pyrolysis temperatures (300, 500, and 800 °C), which were firstly utilized as peroxymonosulfate (PMS) activators for promoting digestate dewaterability with wide applicability. Results showed that compared to the Fe-BC₃₀₀/Fe-BC₅₀₀ + PMS treatments, Fe-BC₈₀₀ + PMS process performed superior digestate dewaterability in which specific resistance to filtration reduction and water content reduction improved by > 12.5% and > 130%, respectively, under the optimal conditions. Mechanistic results demonstrated that in Fe-BC₈₀₀ + PMS system, HO• and SO₄^•- oxidation played a pivotal role on promoted digestate dewaterability, while HO• and ¹O₂ oxidation was dominated in Fe-BC₃₀₀/Fe-BC₅₀₀ + PMS treatments. Fe-BC₈₀₀ containing higher Fe and CO contents could efficiently interact with PMS to generate numerous HO• and SO₄^•- via iron cycle. These highly reactive oxygen species proficiently reduced the hydrophilic biopolymers, protein molecules, and amino acids in extracellular polymeric substances, leading to remarkable decrease in particle size, hydrophilicity, adhesion, network strength, and bound water of digestate. Consequently, the flowability and dewaterability of digestate could be significantly enhanced. The cost-benefit result indicated the Fe-BC + PMS treatment possessed desirable reusability, applicability, and economic viability. Collectively, the Fe-BC + PMS is a high-performance and eco-friendly technique for digestate dewatering, which opens a new horizon towards a closed-loop of digestate reutilization.

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 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.

Application of persulfate-based oxidation processes to address diverse sustainability challenges: A critical review

Over the past years, persulfate (PS) is widely applied due to their high versatility and efficacy in decontamination and sterilization. While treatment of organic chemicals, remediation of soil and groundwater, sludge treatment, disinfection on pathogen microorganisms have been covered by most published reviews, there are no comprehensive and specific reviews on its application to address diverse sustainability challenges, including solid waste treatment, resources recovery and regeneration of ecomaterials. PS applications mainly rely on direct oxidation by PS itself or the reactive sulfate radical (SO₄^•-) or hydroxyl radical (^•OH) from the activation of peroxodisulfate (PDS, S₂O₈^2-) or peroxymonosulfate (PMS, HSO₅^-) in SO₄^•--based advanced oxidation processes (SO₄^•--AOPs). From a broader perspective of environmental cleanup and sustainability, this review summarizes the various applications of PS except pollutant decontamination and elaborates the possible reaction mechanisms. Additionally, the differences between PS treatment and conventional technologies are highlighted. Challenges, research needs and future prospect are thus discussed to promote the development of the applications of PS-based oxidation processes in niche environmental fields. In all, this review is a call to pay more attention to the possibilities of PS application in practical resource reutilization and environmental protection except widely reported pollutant degradation.

Pyrite activated peroxymonosulfate combined with as a physical-chemical conditioner modified biochar to improve sludge dewaterability: analysis of sludge floc structure and dewatering mechanism

In this study, we proposed an advanced oxidation process of pyrite (FeS₂) and peroxymonosulfate (PMS) and prepared a modified polyaluminum chloride biochar (P-BC). The motivation is to use the combination of FeS₂ + PMS + P-BC to improve waste activated sludge (WAS) dewaterability. The method to improve the sludge dewatering effect with the combination of FeS₂ + PMS + P-BC is as follows: in the first step, pour 0.75 g/g TSS FeS₂ and 0.6 g/g TSS PMS into the sludge, and stir for 15 min. Then, add P-BC and stir for 5 min; complete the entire WAS processing process. The vacuum filtration test was used to evaluate the dehydration effect. The water content (Wc) and specific resistance to filtration (SRF) of the raw sludge can be reduced from the original values of 92% and 2.36 × 10¹³ m/kg to 67% and 9.89 × 10¹¹ m/kg, respectively. The results showed that the combination of FeS₂ + PMS + P-BC can effectively improve the sludge dewatering effect through oxidation. A laser particle size analyzer is used to observe changes in sludge particle size. The median diameter of sludge particles increased from 55.37 to 64.56 μm. A zeta analyzer to is used observe changes in sludge zeta potential. The zeta potential of sludge particles increased from - 15.8 to 0.4 mV. In the analysis of extracellular polymeric substances (EPS) of sludge, it was found that protein (PN) and polysaccharide (PS) in EPS decreased significantly. To further analyze the phenomenon of PN and PS drop, excitation-emission-matrix spectra (3D-EEM) was used. To observe the changes of sludge functional group, X-ray photoelectron spectroscopy was used. It was found that FeS₂ + PMS + P-BC can destroy the functional groups of sludge, such as O-H, C-C, and O═C-NH- related to proteins and polysaccharides.

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.

Removal of 4-nonylphenol in activated sludge by peroxymonosulfate activated with sorghum distillery residue-derived biochar

The presence of 4-nonylphenol (4-NP), an endocrine disrupting chemical, waste activated sludge (WAS) or biosolids at elevated content requires effective method for 4-NP reduction in total sludge management. Herein, sorghum distillery residue-based biochar-activated peroxymonosulfate (SDRBC/PMS) system was studied as pretreatment of WAS. Results indicated 91% of 4-NP removal at pH 6.0 in the presence of 3.1 × 10^-6 M and 0.8 g L^-1 PMS and SDRBC500, individually. The synergetic effects of singlet oxygen (¹O₂) and the abundant functional sites (C = O/C-O content) of SDRBC significantly improved 4-NP degradation. The decreased fluorescent dissolved organic matter (DOM) in the sludge also enhanced the pretreatment efficiency. Moreover, the enrichment of the Nitrospira functional bacteria in the microbial community yielded the highest 4-NP degradation in the SDRBC/PMS-pretreated sludge. The SDRBC/PMS system functions mainly via nonradical-mediated oxidation pathway in pretreating WAS in particular and potentially by combined advanced oxidation and biodegradation processes for wastewater treatment in general.

Effects of foreign metal doping on the step-by-step oxidation process in M-OMS-2 catalyzed activation of PMS

Various metal cations M (M = Mg²⁺, Ca²⁺, Zn²⁺, Cu²⁺, Fe³⁺) were doped into the tunnel of manganese octahedral molecular sieve (OMS-2). Redox-inactive metal (Ca, Mg and Zn) doped OMS-2 exhibited better peroxymonosulfate (PMS) catalytic activity than redox metal-doped Cu-OMS-2 and Fe-OMS-2. Redox-inactive metals doping improves the conductivity and reducibility of the catalyst, while transition metal doping reduces the dispersion of manganese. More importantly, the degradation of ACE can be divided into two stages. In the first stage, ACE was oxidized dominantly through mediated electron transfer process. Subsequently, singlet oxygen (¹O₂) gradually dominated oxidative degradation in the second stage, which was derived from the reaction between superoxide radical (O₂^•-) and metastable manganese intermediates. The long half-life of O₂^•- on the surface of OMS-2 ensured the delay generation of ¹O₂. This study not only provides a new idea for improving the efficiency of heterogeneous catalysts activation of PMS, but also meaningful for the in-depth study of multiple reaction mechanisms in PMS activation processes.

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.

Free radicals removing extracellular polymeric substances to enhance the degradation of intracellular antibiotic resistance genes in multi-resistant Pseudomonas Putida by UV/H2O2 and UV/peroxydisulfate disinfection processes

UV-based advanced oxidation processes (UV-AOPs) have been recommended to disinfect wastewater treatment plant (WWTP) effluents to control the dissemination of antibiotic resistance, but the mechanism of intracellular antibiotic resistance genes (i-ARGs) degradation by UV-AOPs is still poorly understood. Here we compared the efficacies of UV, UV/H₂O₂, and UV/PDS in degrading seven i-ARGs carried by a multi-drug resistant P. putida MX-2 isolated from sewage sludge and investigated the roles of free radicals and UV irradiation in degrading the carried i-ARGs in UV-AOPs. The results suggested that although UV/H₂O₂ and UV/PDS were only slightly superior to UV to inactivate P. putida MX-2, they significantly promoted the degradation of i-ARGs. The generated free radicals mainly reacted with the bacterial extracellular polymeric substances (EPS), increased the cell membrane permeability of bacteria, and consequently facilitated UV irradiation enter into the intracellular environment to damage the i-ARGs, thus enhancing their degradation during UV-AOPs processes. Our findings suggested that the removal of bacterial EPS by free radicals greatly contributed to the degradation of i-ARGs by UV irradiation in UV-AOPs, and more efficient approaches that are capable of removing EPS should be further developed to effectively control the dissemination of antibiotic resistance by UV treatment of wastewater effluent.

Understanding the synergistic mechanism of PAM-FeCl3 for improved sludge dewaterability

Hybrid flocculant polyacrylamide-ferric chloride (PAM-FeCl₃) was developed to improve the dewaterability of sewage sludge and the dewatering performance, properties of treated sludge, composition and morphology distribution of extracellular polymeric substance (EPS) were investigated. The physicochemical properties of the PAM-FeCl₃ were characterized, and its effectiveness as a sludge conditioner was evaluated. The results indicated that PAM-FeCl₃ conditioning was able to promote sludge dewaterability. Simultaneously, PAM-FeCl₃ neutralized the negative charges on the surface of sludge particles and increased the sludge floc size. Besides, PAM-FeCl₃ also formed a rough and porous floc structure that reduced sludge compressibility. Meanwhile, the exciting emission matrix analysis suggested that PAM-FeCl₃ can effectively disintegrate of EPS fraction in sludge and decompose the aromatic protein-like substances as well as the humic acid-like substances in EPS. Additionally, the larger sludge floc formation, electrostatic interaction and adsorption bridging effect resulted in compression of sludge structure and the decomposition of EPS fractions and improved sludge dewatering performance.

Recirculation of reject water in deep-dewatering process to influent of wastewater treatment plant and dewaterability of sludge conditioned with Fe2+/H2O2, Fe2+/Ca(ClO)2, and Fe2+/Na2S2O8: From bench to pilot-scale study

Deep dewatering of sewage sludge pretreated with advanced oxidation processes (AOPs) is a strategy for efficient sludge reduction and subsequent disposal. The pretreatment and dewatering performance of sludge conditioned with three types of AOPs (Fe²⁺/H₂O₂, Fe²⁺/Ca(ClO)₂, and Fe²⁺/Na₂S₂O₈), compared with sludge conditioned with traditional conditioner (Fe³⁺/CaO), were investigated in both bench and pilot-scale tests. All of those conditioner systems could reduce the water content of dewatered sludge cake to below 60 wt% in bench-scale (about 16 kg raw sludge per round) and pilot-scale (approximate 800 kg raw sludge per round) diaphragm filter press dewatering. Compared with raw sludge, the deep-dewatering filtrate after different conditioning and dewatering processes had higher ammonia nitrogen (NH₄⁺-N) and chemical oxygen demand (COD) contents due to the degradation of organic matter, and much lower total phosphorus (TP) content due to the formation of iron phosphate precipitate. A better biodegradability (i.e. higher BOD₅/COD ratio) was found in the deep-dewatering filtrate of sludge conditioned with Fe²⁺/H₂O₂ (25.2 %) and Fe²⁺/Ca(ClO)₂ (17.4 %). Most of the heavy metals (Cr, Cu, Ni, and Pb) (>79 wt%) have remained in the dewatered sludge cake, and most of the Cl element (>90 wt%) in the sludge pretreated by Fe²⁺/Ca(ClO)₂ and Fe³⁺/CaO was kept in the filtrate, rather than the dewatered sludge cake. Based on the pilot-scale experimental results, if all the filtrate in the deep-dewatering process returned to the influent of WWTP, the loading ratios of TP, NH₄⁺-N, COD in the four conditioner systems were less than 3 wt%. The above results proved that the AOPs conditioned sludge could achieve deep-dewatering in pilot-scale and the direct recirculation of deep-dewatering filtrate to the influent of wastewater treatment plant was feasible.

Disordered mesoporous carbon activated peroxydisulfate pretreatment facilitates disintegration of extracellular polymeric substances and anaerobic bioconversion of waste activated sludge

The potential of disordered mesoporous carbon (DMC) as catalyst of peroxydisulfate (PDS) to improve sludge solubilization and methane production was investigated. Results showed that DMC activated PDS (DMC/PDS) to produce sulfate radicals (SO₄^-), facilitating cells rupture and sludge matrix dissociation by degrading the carbonyl and amide groups in organic biopolymers (especially proteins, polysaccharides and humus). At the optimal DMC/PDS dosage of 0.04/1.2 g-mmol/g-VS, SCOD was increased from initial 294.0 to 681.5 mg/L, with the methane production rate of 12.6 mL/g-VS/day. Moreover, DMC could serve as electron mediator to accelerate electron transfer of microorganisms, building a more robust anaerobic metabolic environment. Modelling analysis further demonstrated the crucial role of DMC/PDS pretreatment in biological degradation and methane productivity. This study indicated that DMC/PDS pretreatment can prominently enhance the release of soluble substances and methane production, aiding the utilization of PDS oxidation technology for improving anaerobic bioconversion of sludge.

Post-treatment options for anaerobically digested sludge: Current status and future prospect

Anaerobic digestion is the most commonly used sludge treatment technology in large-scale wastewater treatment plants (WWTPs), generating two main products, i.e., biogas and anaerobically digested (AD) sludge. Biogas can be used as a source of renewable energy, and AD sludge is often transported for agricultural land application. Land application of AD sludge is confronted with ever-increasing economic and regulatory pressures due to its high water content, high organic content and related odour and pathogen content (if poorly stabilized), as well as potential toxic metal and organic contaminants. To address these challenges, a number of technologies have been developed for the further treatment of AD sludge before final disposal. This review aims to critically evaluate these state-of-the-art technologies. These technologies were categorized based on their primary aims: 1) dewaterability enhancement; 2) solids reduction and stabilization; 3) toxic metals removal. At present, the goal of post-treatment mainly focuses on dewaterability enhancement, to reduce transport costs. In future, we propose that the post-treatment of AD sludge should orient towards multiple aims, i.e., an integrated approach enabling sludge volume reduction, stabilization (including pathogen removal), and metal solubilization simultaneously. Two promising technical routes are suggested as examples, i.e. physio-chemical iron-based advanced oxidation and biological acidic aerobic digestion, while more approaches need to be developed in future studies. We concluded that post-treatment of AD sludge will promote the AD sludge management towards a more economically favourable, socially acceptable, and environmentally sustainable way; however, further development and rigorous evaluation are required for a wider adoption.

Dewaterability improvement and environmental risk mitigation of waste activated sludge using peroxymonosulfate activated by zero-valent metals: Fe0 vs. Al0

The stabilization and dewaterability of waste activated sludge (WAS) are essential factors for downstream disposal or reuse. Herein, two types of zero-valent metals, zero-valent iron (Fe⁰) and zero-valent aluminum (Al⁰), were compared for their ability to activate peroxymonosulfate (PMS) during the WAS conditioning process, with the effects of PMS activation by these two metals on WAS dewaterability and the potential environmental risks evaluated. Results showed that compared to Al⁰/PMS treatment, Fe⁰/PMS treatment achieved superior WAS dewaterability and reduced operational costs. Using PMS combined with Fe⁰ and Al⁰ treatments under optimal conditions, the water content (Wc) of dewatered sludge decreased to 55.7 ± 2.7 wt% and 59.4 ± 1.3 wt%, respectively. Meanwhile, application of the Fe⁰/PMS treatment system reduced the total annual cost by approximately 33.1%, compared to the Al⁰/PMS treatment. Analysis of the dewatering mechanism demonstrated that in the Fe⁰/PMS treatment, Fe³⁺/Fe²⁺ flocculation played an important role in the enhancement of WAS dewatering, while sulfate radical (SO₄^•-) oxidation was the dominant factor for WAS dewaterability improvement in Al⁰/PMS treatment. The greater enhancement of WAS dewaterability by Fe⁰/PMS treatment, was mainly attributed to more efficient reduction of hydrophilic extracellular polymeric substances (EPS) and an increase in surface charge neutralization. Environmental risk evaluation results indicated that Fe⁰/PMS and Al⁰/PMS treatments both effectively alleviated the environmental risks of heavy metals and faecal coliforms in dewatered sludge. Overall, this study proposes a novel perspective for the selection of an optimal PMS activator in sludge treatment.

Siderite/PMS conditioning-pressurized vertical electro-osmotic dewatering process for activated sludge volume reduction: Evolution of protein secondary structure and typical amino acid in EPS

In this study, the siderite/PMS conditioning-pressurized vertical electro-osmotic dewatering (PEOD) process was used to reduce the volume of activated sludge (AS). The changes in water content, cell, extracellular polymeric substances (EPS) distribution, protein secondary structures and typical amino acids in EPS fractions of AS along siderite/PMS conditioning-PEOD process were investigated. Results showed that the final water content (WC) of dewatered AS was 58.02% under the RSM optimized conditioning conditions of 0.05 g/g TSS siderite dosage, 0.23 g/g TSS PMS dosage, 600 kPa mechanical pressure and 20 V voltage. At conditioning and PEOD stages, the bound water content(BWC) of AS decreased by 25.23% and 91.76%, respectively. The HO• and SO₄^-· generated from siderite activating PMS could lead to the disruption of cells. The ratio of Ala-to Lys (Ala/Lys) showed strong negative correlations with BWC or WC in slime (R_BWC²=-0.803, p<0.01; R_WC²=-0.771, p<0.01) and TB-EPS (R_BWC²=-0.693, p<0.01; R_WC²=-0.705, p<0.01), and could be considered as an indicator of AS dewaterability. Compared with raw AS, conditioning led to the occurrence of the denser protein structure in TB-EPS and the looser one in slime. The contact number between Ala-and water decreased in TB-EPS and increased in slime, which indicated that the migration of water adhered in TB-EPS to outer layer. At the DG, MC and EC process, while the looser protein structure in TB-EPS and the denser one in slime occurred, as well as higher contact number between Ala-and water in TB-EPS than that in slime, which indicated that more water flowed outsider of slime than TB-EPS. This implied that the variations of the compactness of protein secondary structures and the contact number between Ala-and water in EPS layers correlated with AS dewaterability.

Pyrite assisted peroxymonosulfate sludge conditioning: Uncover triclosan transformation during treatment

Waste activated sludge (WAS) dewatering is a crucial process for sludge treatment and disposal. In this study, we proposed a novel pyrite (FeS₂) and peroxymonosulfate (PMS) treatment to improve WAS dewaterability. Micropollutants are commonly enriched in the sludge. It is not clear if the micropollutants remain in the sludge during the conditioning. Triclosan (TCS) as a widely used bactericide often presents in the WAS, thus was chosen as a target micropollutant. Pyrite + PMS treatment could simultaneously enhance WAS dewaterability and TCS removal with low cost and high benefit. Under the optimal conditions, the specific resistance of filtration (SRF) and capillary suction time (CST) were reduced by 84.60% and 74.91%, respectively. Meanwhile, the TCS removal efficiency was 34.08% with four transformation products identified. During the pyrite + PMS process, sulfate radicals and hydroxyl radicals were generated and strong flocculation was induced by iron. These two processes significantly reduced the sticky biopolymers, hydrophilic functional groups, and hydrophilic protein molecular structure of extracellular polymeric substances (EPS), leading to the release of bound water and TCS. Collectively, the pyrite + PMS treatment is a promising alternative for simultaneous enhancement of WAS dewatering and micropollutants removal, which is beneficial to the downstream treatment.

Excess sludge disintegration by discharge plasma oxidation: Efficiency and underlying mechanisms

A huge amount of excess sludge is inevitably produced in wastewater treatment, and it is becoming more and more urgent to realize efficient sludge reduction. Discharge plasma oxidation was used to efficiently disintegrate excess sludge for sludge reduction in this study. Approximately 18.22% sludge disintegration and 27.8% reduction of total suspended solids (TSS) were achieved by discharge plasma treatment. The water content of the filter cake decreased from 81.9% to 76.0% and the bound water content decreased from 2.66 g/g dry solid to 0.73 g/g dry solid after treatment. The large quantities of reactive oxygen species (ROS) generated by discharge plasma played important roles in sludge disintegration by destroying flocs and promoting the transformation of organic substances. Concurrent cell lysis induced by ROS oxidation released intracellular organics and water into the liquid phase. The fraction of soluble extracellular polymer substances (S-EPS) was enhanced from 16.10% to 58.51%, whereas the tightly bound fraction was reduced from 70.62% to 28.91%. Migration and decomposition of EPS were the main processes for EPS changing at a low oxidation capacity, whereas cell lysis became important at a high oxidation capacity. In summary, the plasma treatment effectively improved sludge disintegration.

Improving dewaterability of waste activated sludge by thermally-activated persulfate oxidation at mild temperature

The mass production of waste activated sludge in wastewater treatment plants may lead to environmental pollution and sludge dewatering is an essential process during its treatment. The oxidation of extracellular polymeric substances (EPS) was the core step to achieve deep sludge dewatering. In this study, thermally-activated sodium persulfate (SPS) process was managed to improve the dewaterability of waste activated sludge (WAS) and its internal mechanism was systematically elaborated. Experimental results showed that with 2.0 mmol/g VSS SPS at 80 °C, capillary suction time (CST) was roughly 59.74% of that in raw sludge. Under this condition, 14.66 ± 0.10 × 10¹¹ kg/m of specific resistance to filtration (SRF) and 61.8% ± 0.1% of water content (W_C) was determined, respectively. A solubilization/oxidation process was proposed to unravel the mechanism of the enhanced dewaterability of WAS in thermally-activated SPS process. Mild temperature efficiently disrupted the sludge flocs and broke cell walls, releasing large amounts of EPS into bulk phase. Meanwhile, mild temperature accelerated the decomposition of SPS to generate sulfate radicals (SO₄^-) and hydroxyl radicals (OH) for oxidizing EPS, facilitating the conversion of bound hydrated water into free water and achieving solid-water separation. The higher reaction temperature favored sludge dewatering, whereas overdosing SPS posed no significant impact. Further analysis illustrated that tyrosine protein-like, tryptophan protein-like, fulvic acid-like and humic acid-like substances in various EPS fractions together exerted the influence on sludge dewatering. Furthermore, the synergy process could alter the secondary structure of protein, which caused a loose structure of EPS and the exposure of hydrophobic sites, facilitating the dehydration of sludge flocs. The details of how thermally-activated SPS process enhanced sludge dewaterability provided the theoretical and technical basis for the application of the process under a real-world situation.

What is the role of light in persulfate-based advanced oxidation for water treatment?

Persulfate-based advanced oxidation processes (PS-based AOPs) under UV, visible, or solar irradiation are being intensively investigated for water treatment. Tremendous advances have been made for enhancing the performance towards the destruction of target pollutants, but a deeper understanding of the role of light in different photo-activated PS-based AOPs is still needed as a basis for improving the efficiency. This paper intends to provide an in-depth review of the underlying photo-activation mechanisms and recent progress in various common photo-activated PS-based AOPs reported over the last decade. Based on a comprehensive survey of previous studies, we categorize these processes according to their reaction mechanisms, including activation by direct UV radiation, processes based on dye-photosensitization, activation through ligand-to-metal charge transfer (LMCT), and photocatalytic processes. Moreover, the improvement in performance of contaminant degradation in these processes compared with those in the absence of light are summarized. Finally, we conclude this review by proposing critical challenges and future perspectives for developing efficient photo-activated PS-based AOPs toward improvement in water treatment and remediation.

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.

Improving solid-liquid separation performance of anaerobic digestate from food waste by thermally activated persulfate oxidation

Anaerobic digestion is a promising ecofriendly technology for the management of the continuous increasing food waste (FW). However, the large amount of resulting anaerobic digestate are very difficult to be purified due to high concentration of suspended colloids. Solid-liquid separation is a pivotal step for the subsequent biological treatment of the digestate by activated sludge process. The dewaterability of digestate could directly reflect the solid-liquid separation performance. In this study, a thermally-activated persulfate (PDS) conditioning method was utilized to enhance the digestate dewaterability. Results revealed that PDS thermally conditioning significantly improved the dewaterability by decreasing digestate pH and decomposing organic substances in digestate. The decline of pH, which was resulted from PDS thermally activation reaction, facilitated filterability improvement via reducing the surface negative charges and prompting the oxidizing ability of PDS-relevant radicals. Protein, the main organic component in digestate, was most closely correlated with digestate dewaterability. Fortunately, they were also the most vulnerable constituent under the oxidation attack. PDS thermal conditioning at 80°C was proven to be the most suitable for improving the solid-liquid separation performance of anaerobic. For practical application in conditioning the anaerobic digestate from FW, the conditions should be further optimized according to the digestate characteristic.

Mechanistic insights into a novel nitrilotriacetic acid-Fe0 and CaO2 process for efficient anaerobic digestion sludge dewatering at near-neutral pH

Traditional Fenton or Fenton-like oxidation has been widely studied for waste activated sludge dewaterability. However, the narrow pH range (2.0-4.0) and the instabilities of Fe²⁺ and H₂O₂ have hindered its commercial application. Owing to the high alkalinity of anaerobic digestion (AD) sludge, traditional Fenton or Fenton-like oxidation is economically unfeasible for its dewatering. In this study, we successfully demonstrated a novel and feasible method that used nitrilotriacetic acid (NTA)-Fe⁰ combined with CaO₂ (NTA-Fe⁰/CaO₂) at near-neutral pH (∼6.0) (a slight pH adjustment) in which capillary suction time ratio (CST₀/CST) and centrifuged weight reduction (CWR) improved by 6 folds and 42.98 ± 0.37%, respectively, under the optimal conditions. The presence of NTA accelerated the Fe⁰ corrosion, Fe²⁺ stability and turnover between Fe²⁺ and Fe³⁺. As such, Fe⁰ could effectively catalyze CaO₂ to produce hydroxyl radicals (•OH) under near-neutral conditions. Accordingly, various molecular weight hydrophilic compounds in different extracellular polymeric substances fractions were significantly reduced after treatment. The hydrophilic functional groups especially protein molecules were largely reduced. Consequently, the viscosity of sludge and particle size effectively decreased, while the release of bound water, surface charge, flocculation, and flowability of sludge were improved. The cost-benefit analysis further demonstrated the NTA-Fe⁰/CaO₂ treatment has high reusability and stability and is also more economical over the FeCl₃/CaO and Fenton's reagent/CaO treatments. In summary, the NTA-Fe⁰/CaO₂ process is a cost-effective and practically feasible technology for improving AD 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.

Enhancement of fermentative volatile fatty acids production from waste activated sludge by combining sodium dodecylbenzene sulfonate and low-thermal pretreatment

This study investigated the combination of low-thermal and sodium dodecylbenzene sulfonate (SDBS) as a pretreatment method to improve the volatile fatty acids (VFA) production from waste activated sludge (WAS). The results showed that the maximum VFA yield of 320 ± 7.7 mg COD/g VS was obtained in the combined pretreatment (0.01 g SDBS/g TS + 70 °C for 60 min), which was 1.8, 1.7 and 4.0 times of that from sole low-thermal, sole SDBS and the control test. The mechanism study revealed the combined pretreatment had synergetic effect on enhancement of disintegration of WAS. Moreover, low-thermal exhibited greater performance on releasing organic matters, and SDBS accelerated hydrolysis and acidogenesis, thus contributing to the enhancement of VFA production in the combined pretreatment. The microbial community analysis demonstrated that the combined pretreatment increased the abundance of phyla Firmicutes which might be responsible for the improvement of VFA production.

Cohesive strength changes of sewer sediments during and after ultrasonic treatment: The significance of bound extracellular polymeric substance and microbial community

Sewer flushing is widely used to remove sewer sediment from drainage systems; however, its performance and cleaning efficiency are limited by the cohesive strength of sewer sediment. To address this, ultrasound, as a clean technology, is proposed to reduce the cohesive strength of sewer sediment. This study investigated the variations in the cohesive strength, extracellular polymeric substances (EPSs), and microbial community of sewer sediment with ultrasonic treatment. During ultrasonic conditioning, the degradation process of the cohesive strength followed the first-order kinetic model and was positively related to the degradation of bound-EPSs. Field emission scanning electron microscopy, particle diameter, and three-dimensional excitation emission analyses suggested that ultrasound reduced the cohesive strength by decreasing the bound-EPS concentration, which reduced the particle size of sewer sediment, and by destroying the structure of tryptophan proteins, which impaired the stability of agglomerated particles. Following ultrasonic treatment, the cohesive strength of the treated sediment was reduced to 69.3% of that of the raw sewer sediment after storage for 21 days; this result could be ascribed to the improvements in polysaccharide transport, amino acid transport, and the cell wall biogenesis functions of the microbial community, as indicated by PICRUSt. Furthermore, next-generation sequencing studies suggest that the proportions of Syntrophomonadaceae, Bacteroidetes_vadinHA17, Synergistaceae, and Syntrophaceae, which are associated with anaerobic digestion and methane production in sediment, improved conspicuously after ultrasonic conditioning.

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.