Improving Sludge Dewaterability Research Articles

A combined ultrasonic and chemical conditioning process for upgrading the sludge dewaterability

ABSTRACT Dewatering is of paramount step in sludge management, and conditioning with coagulant is considered as the commonest way to improve the sludge dewaterability. Sludge disintegration process was examined through Ultrasound (US) to strip the water from the sludge flocs and accordingly four coagulants: aluminium sulphate (Alum), iron (III) chloride (FeCl3), cationic polyacrylamide (CPAM) and combined ferric chloride and lime (CC) with optimum dosages were employed to improve the sludge dewaterability in terms of capillary suction time (CST), specific resistance to filtration (SRF) and moisture content (MC). The ultrasonic sludge disintegration with 1 min exposure with an energy dosage of 700 kJ/kg TS realises the best sludge dewatering efficiency. In addition, the effects of CPAM and CC conditioning on sludge dewaterability were higher compared with two other conditioners. The combined US-CPAM and US-CC with optimum dosages 8 and 101 mg/kg DS significantly improved sludge dewaterability parameters (MC: 68.50%, SRF: 1.03 × 1010 m/kg and CST: 31.82 s) and (MC: 72%, SRF: 1.08 × 1010 m/kg and CST: 31.82 s) compared with when conditioning experiments were performed alone. Scanning electron microscopy (SEM) images showed that the combined US-CPAM and US-CC conditioning formed a more porous structure and low compressible sludge filter cake which are desired to sludge dewatering. Therefore, the ultrasonic-assisted conditioning with CPAM and combined coagulant (lime + Ferric chloride) could significantly improve the sludge dewaterability indices.

Full-scale investigation of ferrous dosing in sewers and a wastewater treatment plant for multiple benefits

This study demonstrates the full scale application of iron dosing in a metropolitan wastewater treatment plant (WWTP) and the upstream sewer system for multiple benefits. Two different dosing locations, i.e., the WWTP inlet works (Trial-1) and upstream sewer network (Trial-2) were tested in this study. Both dosing trials achieved multiple benefits such as sulfide control, phosphate removal and improved sludge dewaterability. During Trial-1, a sulfide reduction of >90% was achieved at high dosing rates (>19 kgFe ML−1) of ferrous chloride in the inlet works and in Trial-2 the in-sewer ferrous dosing had significant gas phase hydrogen sulfide (H2S) concentration reduction in the sewer network. The ferrous dosing enhanced the phosphate removal in the bioreactor up to 76% and 53 ± 2% during Trial-1 & 2, respectively. The iron ending up in the anaerobic sludge digester reduced the biogas H2S concentration by up to 36% and 45%, respectively. The dewaterability of the digested sludge was improved, with relative increases of 9.7% and 9.8%, respectively. The presence of primary clarifier showed limited impact on the downstream availability of iron for achieving the afore-mentioned multiple benefits. The iron dosing enhanced the total chemical oxygen demand removal in the primary clarifier reaching up to 49% at the high dose rates during Trial-1 and 42 ± 1% during Trial-2. This study demonstrated that multiple benefits could be achieved independent of the iron dosing location (i.e., at the WWTP inlet or in the network). Further, iron dosing at both locations enhances primary settling, beneficial for bioenergy recovery from wastewater.

Waste activated sludge conditioning in a new Fe2+/persulfate/tannic acid process: Effectiveness and optimization study to enhance dewaterability

Fe2+/persulfate process can be used to improve sludge dewaterability significantly. However, a large dose of chemicals was always needed and it will cause filter cloth clogging, equipment corrosion and sludge calorific value reduction in practice. In this study, a new Fe2+/persulfate/TA (tannic acid) process was put forward, which could reduce the use of inorganic reagents and keep the outstanding conditioning capability. A Box-Behnken experimental design method based on response surface methodology (RSM) was selected to assess the roles of independent variables including SPS (sodium persulfate), Fe2+ and TA, on sludge dewaterability characterized by capillary suction time, specific resistance of filtration, and water content of dewatered sludge cake. The optimized conditions were SPS dosage of 0.43 mmol/gTS (total solid), Fe2+ dosage of 0.58 mmol/gTS and TA dosage of 0.14 mmol/gTS, at which the water content of dewatered sludge cake showed (75.89 ± 0.01)% highly close to the predicted minimum water content of dewatered sludge cake using the fitted equation. Moreover, in the new process, sludge properties altered distinctly with the elimination of extracellular protein, supernatant viscosity reduction and particle size increase. Further investigations were needed to explore the underlying mechanism and practical application for a better and comphrensive understanding of Fe2+/SPS/TA process.

Sludge predation by aquatic worms: Physicochemical characteristics of sewage sludge and implications for dewaterability

Worm predation is an eco-friendly and low energy cost means of sludge reduction that has shown great promise. The worm predation process consists of inoculating specific species of aquatic worms into the sludge system to build a suitable biotope. After predation, the residual sludge needs to be discharged and disposed. This study investigates the potential effects of worm predation on sludge dewatering focusing on the mechanisms, characteristics of sedimentation, filtration and dewatering for the sludge from the predation process. It also describes the changes in extracellular polymeric substance and bound water, and discusses options to improve sludge dewatering. The results suggested that worm predation restrained the viscosity and surface charge of residual sludge and helped prevent sludge bulking. However, sludge filterability deteriorated when predation occurred. The residual sludge experienced a significant reduction in the contents of loosely bound and tightly bound extracellular polymeric substances and a decreased ratio of protein to polysaccharides content during the five days of predation. Extending the sludge retention time of predation to five days contributed to a release of bound water and synergistically improved centrifugal dewatering of the residual sludge with ferric chloride added as conditioner.

Experimental investigation on sludge dewatering using granulated blast furnace slag as skeleton material.

The highly compressible nature of sludge and the presence of colloidal particles cause difficulties in sludge dewatering. Reducing the moisture content in secondary sludge is a key factor in reducing the capital costs, operational costs, and transportation costs in wastewater management. This investigation concerned the combined utilization of quicklime and granulated blast furnace slag (GBFS) to improve sludge dewatering. The experimental work included the initial characterization of the sludge and granulated blast furnace slag and evaluation of the dewatering ability of the treated sludge (CST, moisture content, turbidity, zeta potential, and heavy metal and biopolymer contents). Optimization using the Box-Behnken design (BBD) was carried out with various operational parameters, and the best performance was found to be at a pH of 10.2, a dose of 0.34g/g DS, and a contact time of 14min. A characterization study was carried out by scanning electron microscopy (SEM) in conjunction with EDS, X-ray diffractometry (XRD), and Fourier transform infrared spectroscopy (FTIR) to confirm the structural features (dense), elemental composition, and the presence of different functional groups. Hence, this study concluded that the use of quicklime with granulated blast furnace slag is suitable for conditioning during sludge dewatering. Graphical abstract.

New insights into the effect of sludge proteins on the hydrophilic/hydrophobic properties that improve sludge dewaterability during anaerobic digestion

Extracellular polymer proteins have been reported to play an important role in enhancing sludge dewaterability during anaerobic digestion in our previous study. However, how the proteins in sludge determine sludge dewaterability remains to be determined. In this work, proteins from digested sludge were identified using label free proteomics analysis, and its hydrophilicity/hydrophobicity properties and functional groups were analysed. We determined that the microbial community variation between the three stages during the anaerobic digestion process was responsible for enhancing sludge dewaterability; The transformation from hydrophilicity to hydrophobicity of digested sludge surface is the result of functional groups distribution variation which caused by the proteins and microbial communities. This study provides a new insight into the development of anaerobic digestion based on sludge dewaterability.

Enhancement of the sewage sludge dewaterability by using ethanol and Fe(III)-rice husk.

Proteins of extracellular polymeric substances (EPS) in sewage sludge play a key role in the sludge dewatering. Ethanol can denature proteins and improve sludge dewaterability. In this study, ethanol was used to precondition and combined with Fe3+ and rice husk (RH) for dewatering enhancement. The experimental results of the capillary suction time (CST) reduction efficiency indicated that the sewage sludge pretreated with ethanol and Fe3+-RH revealed well cooperative formation mechanism with regard to dewatering performance. Using the response surface methodology (RSM) determined that CST reduction efficiency of sewage sludge reached 78.5% under optimal conditions of ethanol 25.21 g/g dry solid (DS), Fe3+ 185.70 mg/g DS, and RH 406.02 mg/g DS, respectively. Moreover, the results showed that the composite conditioner is effective for the specific resistance to filtration decreased from initial 1.66E + 13 m/kg to 2.44E + 11 m/kg. The analysis of EPS showed that extractable proteins in EPS increased to maximum when the sludge was pretreated by Fe3+-RH because EPS were destroyed and proteins in EPS were released. After the addition of ethanol, extractable protein content was reduced because of protein precipitation and released interstitial water and bound water. The sludge morphology analysis indicated that the RH as a skeleton builder provided the outflow passages, which enhanced the dewatering performance of sludge. From these results, the combination treatment of ethanol and Fe3+-RH is a promising synergetic strategy to enhance the dewaterability of sewage sludge.

Impact of initial sludge pH on enhancing the dewaterability of waste activated sludge by zero-valent iron-activated peroxydisulphate

ABSTRACT In this study, the usefulness of the system in enhancing the dewaterability of waste activated sludge was studied at different initial sludge pH levels to reveal the associated mechanisms. Results showed that conditioning of sludge by the system at acidic initial sludge pHs enhanced sludge filterability and the dryness of dewatered sludge cake, while only the sludge filterability was improved at neutral initial sludge pH. In particular, the conditioning treatment using 0.353 g/g DS of ZVI and Na2S2O8 with a mole ratio of 1.25:1, at initial sludge pH 2.52, was the optimum condition to improve sludge dewaterability, which reduced the moisture content of dewatered sludge cake to only 69.8% and meanwhile reduced sludge CST and SRF to only 70.9% and 40.7% of that of raw sludge, respectively. During the conditioning treatment, sludge microbial cell lysis resulting from decreasing the initial sludge pH led to the reduction in the moisture content of dewatered sludge cake, while the oxidation of sludge EPS and the coagulation of the disrupted sludge flocs achieved by the system improved sludge filterability. A kaolin suspension experiment revealed that with a decrease in system pH, the oxidation effect was gradually inhibited and the coagulation effect offset the disruption effect on sludge flocs to improve the filterability of sludge. Therefore, the conditioning of waste activated sludge using the ZVI/Na2S2O8 system at acidic initial sludge pHs is useful to enhance the sludge filterability and the dryness of dewatered sludge cake, both of which are crucial for improving sludge dewatering performance.

Conditioning with zero-valent iron or Fe2+ activated peroxydisulfate at an acidic initial sludge pH removed intracellular antibiotic resistance genes but increased extracellular antibiotic resistance genes in sewage sludge

Sulfate radical (SO4−)-based conditioning methods, such as zero-valent iron (ZVI, i.e., Fe0) or ferrous iron (Fe2+) activated peroxydisulfate (S2O82-), have recently developed to improve sludge dewaterability, but it remains unclear how they impact the intracellular and extracellular antibiotic resistance genes (ARGs) in sewage sludge. In this study, it was found that conditioning treatments that used ZVI/S2O82- or Fe2+/S2O82- system, at an acidic initial sludge pH, removed the intracellular ARGs and intI1 and the extracellular intI1 from sewage sludge, but led to the accumulation of extracellular ARGs of aadA-01, aadA-02, aadA1, aadA2-03, and strB in conditioned sludge. During sludge conditioning with ZVI/S2O82- or Fe2+/S2O82-, bacterial hosts of ARGs and intI1 were seriously lysed to release the intracellular ARGs and intI1 to the extracellular environment, thus removing intracellular ARGs and intI1 in sludge, while the released ARGs and intI1 were primarily degraded by the produced SO4− to attenuate most extracellular ARGs and intI1. However, the relatively lower degradation ability of SO4− for extracellular ARGs of aadA-01, aadA-02, aadA1, aadA2-03, and strB led to their accumulation in conditioned sludge. Therefore, SO4−-based conditioning methods can be employed to reduce ARGs in sludge, but the subsequent treatment of sludge dewatering filtrate requires more attention.

Micro-interfacial mechanisms on sludge dewaterability enhancement using cerium chloride for preparation of carbon-based functional material

In this work, sludge conditioning efficiency of cerium chloride (CeCl3) in combination with organic polymers was evaluated, the floc microstructure and extracellular polymeric substances (EPS) properties under flocculation conditioning were analyzed. The interaction mechanisms between EPS and Ce(III) were systematically investigated through two-dimension correlation spectroscopy, X-ray photoelectron spectroscopy, and confocal laser scanning microscopy. In addition, the adsorption and catalytic abilities of Ce-sludge based carbon (SBC) in tetracycline (TC) removal were evaluated. The results showed that CeCl3 conditioning performed well in improving sludge dewaterability, and CeCl3 and cationic polyacrylamide showed a synergistic effect in sludge conditioning. Contents of EPS decreased as the dosage of CeCl3 increased because of charge neutralization and complexation reactions. 2D-UV-FTIR heterospectral correlation spectroscopy analysis suggested the reaction activity of EPS to Ce(III) followed the order of humic acid > protein > polysaccharide. The decrease of α-helix content improved the hydrophobicity of proteins in EPS, which was responsible for sludge dewaterability improvement in CeCl3 conditioning. Besides, the SBC was prepared with CeCl3 conditioned sludge for treating water containing TC. Ce-SBC had a dual function of adsorption and Fenton-like activity. This work provides a sludge recycling process that coupled chemical conditioning to pyrolysis carbonization to prepare functional carbon-based materials.

Improvement of the Dewaterability of Excess Activated Sludge with Mixed Yeasts by Degrading Extracellular Polymeric Substances

The dewaterability of excess sludge directly affects the efficiency and cost of sludge disposal, and improving sludge dewaterability is a crucial way to reduce sludge volume. This study proposes a method to improve the dewaterability of residual sludge by using mixed yeast strains to degrade extracellular polymeric substances (EPS) in activated sludge. Firstly, the mixed cells of three yeast strains were injected into the sterilized EPS solution to investigate the degradation efficiency of EPS components. Secondly, the mixed yeast cells were supplied into the residual sludge, which was aerated for several hours while the sludge dewaterability was evaluated. The results showed that the degradation efficiencies of yeast to proteins, polysaccharides, and nucleic acids in EPS were evident, and reductions of (60.43±2.73)%, (18.94±2.39)%, and (48.30±3.37)% were achieved, respectively, within 72 hours' oscillating cultivation. The capillary suction time (CST) of the sludge decreased by (17.19±1.16)% after aeration, with 1.5 g mixed yeast wet cells added into 2 L excess sludge, (7.03±1.35)% more than that of the control test after 24 hours. Meanwhile, the total amount of EPS in sludge decreased by (17.46±3.91)% more than that in the control sludge, indicating that the yeast can improve the sludge dewaterability in-situ by degrading EPS in sludge.

Relationship between the physicochemical properties of sludge-based carbons and the adsorption capacity of dissolved organic matter in advanced wastewater treatment: Effects of chemical conditioning.

Pyrolysis carbonisation is a promising technology to convert organic waste into valuable carbon-based materials. However, sludge is generally highly compressible and difficult to dewater because of its high concentrations of biopolymers; the bound water of sludge is trapped in a network composed of biopolymers. Therefore, chemical conditioning is an indispensable step for improving sludge dewaterability performance. In the present work, the effects of different chemical conditioning agents (polymeric aluminium chloride (PACl), iron(III) chloride (FeCl3), KMnO4-Fe(II) and Fenton's reagent) on the physicochemical properties of sludge-based carbons (SBCs) were systematically studied and the SBCs were further used in advanced wastewater treatment. The adsorption mechanisms of dissolved organic matters (DOMs) by different SBCs were also investigated. The results showed that conditioning with KMnO4-Fe(II) and Fenton's reagent improved the specific surface area of the SBCs, whereas inorganic salt flocculation conditioning reduced the porosity of the SBCs. In addition, we found that the Fenton-SBC and Mn/Fe-SBC performed better than the other investigated SBCs in the removal of organic compounds from secondary effluent and that the pseudo-second-order kinetic model could better describe the process of DOMs adsorption by all of the investigated SBCs. Moreover, three-dimensional fluorescence excitation-emission matrix spectroscopy in combination with an analysis of the physical and chemical fractionation of DOMs showed that all of the SBCs performed well in the adsorption of aromatic substances, hydrophobic acids and hydrophobic neutrals, whereas the Mn/Fe-SBC and Fenton-SBC performed better than the other SBCs in the removal of weakly hydrophobic acids.

Drained water quality in sludge treatment wetlands: Effects of earthworm densities and plant species

Earthworms can improve sludge dewatering and stabilization in sludge treatment wetlands (STWs). However, drained water quality in STWs with earthworm addition is not well known. In this study, the combination of two plant species (Phragmites australis and Typha angustifolia) and earthworm (Eisenia foetida) densities of 2.7, 5.4, 8.1, 10.8, 13.5 and 16.2 kg/m3 in six STWs were investigated to evaluate their effects on drained water quality. Meanwhile, clogging characterization of the six STWs was evaluated. The results showed that filtration rates in the three earthworm STWs were 0–0.4 cm/s higher than those in the STWs without earthworm addition. P. australis and T. angustifolia had positive effects on chemical oxygen demand (COD), total nitrogen (TN), total phosphorus (TP) and ammonium (NH4+) removal, with removal efficiencies all over 80%. Moreover, NH4+ removal efficiency was significantly different between P. australis and T. angustifolia; the higher NH4+ removal efficiencies of 89.5–90.4% were determined in the two P. australis STWs. Meanwhile, canonical correlation analysis and principal component analysis indicated that the combination of plants and earthworms was beneficial for drained water treatment in the STWs. Furthermore, the optimum drained water treatment was determined in the P. australis STWs with an earthworm density of 10.8 kg/m3, with the mass removal efficiencies for COD, NH4+, TN and TP being 99.1%, 93.1%, 91.5% and 91.0%, respectively. Thus, it can be concluded that P. australis and earthworm addition improved drained water quality and alleviated clogging in the STWs.

Performance and mechanisms of wastewater sludge conditioning with slag-based hydrotalcite-like minerals (Ca/Mg/Al-LDH)

Extracellular polymeric substances (EPS) in wastewater sludge form a network structure that is highly hydrophilic and compressible. Thus chemical conditioning is always required to improve sludge dewaterability by changing the gelatinous structure of sludge flocs. Layered double hydroxides (LDH) are generally characterized by large surface area and high anion exchange capacity, so we prepared three types of hydrotalcite-like compounds (Ca/Mg/Al-LDHs) from a typical solid waste, blast furnace slag, using NaOH precipitation (giving LDHa), a hydrothermal method (LDHb), and NaOH–Na2CO3 precipitation (LDHc). The physicochemical properties of the three LDH were comprehensively characterized, and their effectiveness as sludge conditioners was evaluated. The results showed that LDH conditioning was able to promote sludge dewaterability, and conditioning efficiency was strongly dependent on LDH structural properties. LDH neutralized the negative charges onto sludge particles and interacted with EPS to increase floc strength. LDH also formed a skeletal structure that reduced sludge compressibility. In addition, there were interactions between the LDH surfaces and the OC–OH in EPS proteins, which altered the secondary structure of protein molecules, consequently increasing sludge dewaterability. The biomolecules of low-molecular-weight fractions (such as peptides and humic acids) in soluble EPS intercalated LDH. Both the surface complexation of organic matter containing carboxyl groups and the intercalation of small molecules in soluble EPS were responsible for EPS–LDH interactions. The combination of skeleton formation, electrostatic interaction, and EPS–LDH interactions resulted in compression of gel-like structure and improved sludge dewatering performance. We finally suggested a novel sludge treatment process that increases sludge dewaterability using slag-derived Ca/Mg/Al-LDH to condition the sludge, and it could be combined with pyrolysis to prepare multi-functional materials or bio-oil.

Novel insight into sludge dewaterability mechanism using polymeric aluminium ferric chloride and anaerobic mesophilic digestion treatment under ultrahigh pressure condition

There are significant challenges associated with treating sewage sludge with high water content. In this study, polymeric aluminium ferric chloride (PAFC)/anaerobic mesophilic digestion (AMD) conditioning combined with ultrahigh pressure filter press (UPFP) dewatering was investigated as a novel and feasible technique to improve sludge dewatering. Response surface methodology experiments found that the optimal operating conditions were at an AMD temperature of 42.6 °C, an AMD reaction time of 240.00 min, a PAFC dose of 150.00 mg/g dry solid, and filter pressure of 5.37 MPa. Under these optimal conditions, the water content of the dewatered cake was reduced by 54.54 wt%. The study explored the detailed mechanisms involved with the conditioning and dewatering process, by tracing the sludge, filter cloth, and filtrate characteristics. First, during the combined PAFC/AMD conditioning process, the treatment effectively removed the protein-like and polysaccharide-like materials in the extracellular polymeric substances of sludge, forming large flocs. Over time, the sludge had a lower viscosity and a higher flowability, promoting the release of bound water. Second, during the UPFP dewatering process, the filter cloth maintained its high permeability, and water easily passed through it. Third, high levels of dissolved organic matter were released after the dewatering process, ultimately forming a porous and rigid sludge cake. Furthermore, the study found strong correlations between sludge dewaterability and sludge, filter cloth, and filtrate characteristics. The experiment involving recycling filter cloth demonstrated that the combined treatment effectively prevented and reduced clogging of the filter cloth, reducing costs.

Sludge dewaterability: The variation of extracellular polymeric substances during sludge conditioning with two natural organic conditioners

To assess the characteristics of Extracellular Polymeric Substances during natural organic conditioners for sludge treatment, Moringa oleifera (MO) and chitosan (CTS) were used as conditioners. The findings of this study show that despite the high EPS content due to the fact that this component is not destroyed upon conditioning, sludge conditioned with MO and CTS displayed efficient and improved dewaterability. MO and CTS showed the same mechanism of action by protonation of the negatively charged EPS and reduction of electrostatic repulsions between sludge flocs, thus enhancing sludge filterability by neutralizing and settling. The effect of MO and CTS on EPS lies in their ability to neutralize EPS, settling them. MO and CTS neutralize and aggregate the EPS, hence improving sludge dewaterability. The results indicate that CTS can impact the EPS quantity and quality while MO impacts the EPS quality. Understanding the impact of MO and CTS on sludge EPS can help elucidate the mechanism of their dewaterability efficiency.

Iron powder activated peroxymonosulfate combined with waste straw to improve sludge dewaterability

ABSTRACT In an activated sludge system, the high hydrophilicity of extracellular polymeric substances (EPS) and the high compressibility of sludge greatly hinder sludge dewatering. A new method for improving the dehydration of waste activated sludge was explored by using iron powder activated peroxomonosulfate combined with waste straw (Fe0+PMS + WS). Specific resistance to filtration (SRF) and water content (Wc) were used to characterize the dewatering performance of sludge. Under the optimal measurement, Wc and SRF were significantly reduced. To reveal the synergistic effect of this joint treatment, zeta potential, particle size distribution, three-dimension excitation emission matrix (3D-EEM) fluorescence spectroscopy, bound water content analysis, and scanning electron microscopy (SEM) were used to investigate the mechanism of sludge dewatering. Results showed that the tightly bound EPS (TB-EPS) was oxidized by sulfate radicals () to loose bound EPS (LB-EPS) and soluble EPS(S-EPS). SEM analysis displayed that the Fe0+PMS + WS combination regulated the formation of a more porous sludge filter cake structure. In addition, the low calorific value of the dewatered sludge after 12 h in open air was significantly increased, and the Wc of the dewatered sludge cake was reduced to 25%. These parameters were beneficial to the subsequent disposal of sludge.

New insight on the combined effects of hydrothermal treatment and FeSO4/Ca(ClO)2 oxidation for sludge dewaterability improvement: From experimental to theoretical investigation

A combination of hydrothermal treatment and FeSO4/Ca(ClO)2 oxidation was developed to enhance the dewatering performance of sludge. First, capillary suction time (CST), specific resistance to filterability (SRF), and water content (Wc) are used to evaluate the sludge dewaterability. The effect of the hydrothermal temperature, the concentration and the molar ratio of FeSO4 and Ca(ClO)2 on the sludge dewatering performance was examined to determine the optimal conditions. Furthermore, the mechanism of the degradation of EPS in sludge was studied using density functional theory (DFT), e.g., electrostatic potential (ESP) analysis, bond order analysis, and bond dissociation enthalpy (BDE) analysis. The results show that the sludge dewaterability was significantly improved with the combined method. The floc structure and the oxygen-containing groups of EPS in sludge are the key reasons for its hydrophilicity and high water content. The combined method can enhance sludge dewatering by breaking the floc structure and removing oxygen functional groups, which is a promising technology for sludge dewatering.

Behaviors of dewaterability and heavy metals of waste activated sludge conditioned by heat-activated peroxymonosulfate oxidation

In this work, heat-activated peroxymonosulfate (heat-PMS) oxidation was used to condition waste activated sludge. The results showed that the optimal temperature and PMS dosage for sludge dewatering were 75 °C and 150 mg/g-volatile solids (VS), and the addition of rice husk (100–400 mg/g-VS) was favorable to further enhance sludge dewaterability. Under optimal conditions, the capillary suction time reduction and water content of sludge cake were 82.4% and 65.8%, respectively. The significant improvement of sludge dewaterability was ascribed to the releases of extracellular polymeric substances (EPS)-bound water and cell water caused by the significant breakdown of tightly bound EPS, as well as the skeleton effect. In addition, heat-PMS oxidation has been demonstrated with the merits of improving the immobilization of heavy metals (HMs) and weakening their environmental risk. After conditioning, HMs solubilization was enhanced significantly and their leaching toxicity decreased obviously. Meanwhile, HMs were transformed into more stable forms. Therefore, heat-PMS oxidation might be a potential and desirable technology for sludge conditioning with respect to the enhancement of sludge dewaterability and the decrease of HMs environmental risk.

Improving sewage sludge dewaterability with rapid and cost-effective in-situ generation of Fe2+ combined with oxidants

Zero-valent iron (ZVI) combined with oxidants is a promising conditioning strategy for improving sewage sludge dewaterability. However, challenges, such as a low dewatering rate and utilization efficiencies of ZVI, restrict further application of ZVI/oxidant treatments. This study employs, for the first time, rapid and cost-effective in-situ generation of Fe2+ combined with common oxidants, namely H2O2, Na2S2O8, and KMnO4, to effectively improve sludge dewaterability. A comparative investigation of a standalone ZVI/oxidants treatment and when combined with a thermal and acid-wash (TA) pretreatment (TA/ZVI/oxidants) was systematically conducted to determine the sludge dewatering efficiency. Under the optimal operational conditions, the average water content of dewatered cake (Wc) was 18.47% less with the TA/ZVI/oxidants treatment when compared with the ZVI/oxidants treatment. The Wc values decreased to 54.81, 58.06, and 59.32 wt%, after the TA/ZVI/H2O2, TA/ZVI/Na2S2O8, and TA/ZVI/KMnO4 treatments, respectively. The potential mechanism by which the TA/ZVI/oxidants treatment enhanced dewatering was investigated by determining the characteristics of both the sludge and the ZVI surface structure. Results indicate that the TA/ZVI/oxidants process caused more Fe2+ dissolution, more strong oxidizing radicals and more strong flocculating Fe3+. The large amount of strong oxidizing radicals and strong flocculating Fe3+ caused cell lysis and disrupted the protein-like and polysaccharide-like materials in the EPS, thereby decreasing the particle size, negative zeta potential, viscosity and colloidal forces, which then released the bound water trapped within the net-like floc and led to improved sludge dewaterability. In the ZVI reuse experiment, the TA/ZVI/oxidants treatment remained highly effective for sludge dewatering even after four cycles. Furthermore, economic analysis indicates that the TA/ZVI/H2O2 treatment can save approximately 52.32% per year compared with using the traditional polyacrylamide (PAM) treatment.