Insights into conditioning of landfill sludge by FeCl3 and lime

Mechanistic insights into melanoidins-induced hydrophilicity of thermal hydrolyzed sludge and its impact on dewaterability

Although thermal hydrolysis pretreatment enhances disposal efficiency of sludge, it inevitably leads to melanoidins formation, which will negatively impact the subsequent wastewater treatment processes. However, their effect on the dewaterability of thermal hydrolyzed sludge (THS) remains poorly understood. This study aimed to uncover the underlying mechanisms of how melanoidins affecting dewaterability of THS. Using resin-adsorption method to reduce melanoidins content by 50% led to 21% and 6% decreases in capillary suction time and specific resistance to filtration, respectively, and a 14% increase in sludge cake solid content. Conversely, accumulating melanoidins to 200% worsened THS dewaterability, altering surface morphology and reducing floc stability, which increased the content of bound water by 9%. Additionally, a higher melanoidins level increased the hydrophilic components in extracellular polymeric substances while reducing hydrophobic sites and structures. These findings indicate that melanoidins impair THS dewaterability by altering flocs spatial properties and increasing hydrophilic structures and components.

Revealing the interplay of dissolved organic matters variation with microbial symbiotic network in lime-treated sludge landscaping

Lime pretreatment is commonly used for sludge hygienization. Appropriate lime dosage is crucial for achieving both sludge stabilization (lime dosage >0.2 g/g-TS) and promoting plant and soil health during subsequent landscaping (lime dosage <0.8 g/g-TS). While much research has been conducted on sludge lime treatment, few studies have examined the effects of lime dosing on integrating sludge stabilization and plant growth promotion during landscaping. In this study, we investigated microbial dynamics and dissolved organic matter (DOM) transformation during sludge landscaping with five lime dosage gradients (0, 0.2, 0.4, 0.6, 0.8 g lime/g-TS) over 90 days. Our results showed that a lime dosage of 0.4 g/g-TS is the lower threshold for achieving waste activated sludge (WAS) stabilization during landscaping, leading to maximum humic substance formation and minimal phytotoxicity. Specifically, at 0.4 g/g-TS lime dosage, protein degradation and decarboxylation-induced humification were significantly enhanced. The predominant microbial genera shifted from Aromatoleum to Exiguobacterium and Romboutsia (both affiliated with the phylum Firmicutes). Reactomics analysis further indicated that a 0.4 g/g-TS lime dosage promoted the hydrolysis of proteins (lyase reactions on C-C, C-O, and C-N bonds), amino acid metabolism, and decarboxylation-induced humification (e.g., C1H2O2, C2H4O2, C5H4O2, C6H4O2). The co-occurrence network analysis suggested that the phyla Firmicutes, Proteobacteria, and Bacteroidetes were key players in DOM transformation. This study provides an in-depth understanding of microbe-mediated DOM transformation during sludge landscaping and identifies the optimal lime dosage for improving sludge landscaping efficiency.

Leaching behavior of microplastics during sludge mechanical dewatering and its effect on activated sludge

Dewatering is an indispensable link in sludge treatment, but its effect on the microplastics (MPs) remains inadequately understood. This study investigated the physicochemical changes and leaching behavior of MPs during the mechanical dewatering of sludge, as well as the impact of MP leachates on activated sludge (AS). After sludge dewatering, MPs exhibit rougher surfaces, decreased sizes and altered functional groups due to the addition of dewatering agents and the application of mechanical force. Meanwhile, plastic additives, depolymerization products, and derivatives of their interactions are leached from MPs during sludge dewatering process. The concentration of MP-based leachates in sludge is 2-25 times higher than that in water. The enhancement of pH and ionic strength caused by dewatering agents induces the release of MP leachates enriched with protein-like, fulvic acid-like, and soluble microbial by-product-like substances. The reflux of MP leachates in sludge dewatering liquor to the wastewater treatment system negatively impacts AS, leading to a decrease in COD removal rate and inhibition of the extracellular polymeric substances secretion. More importantly, MP leachates cause oxidative stress to microbial cells and alter the microbial community structure of AS at the phylum and genus levels. These findings confirm that MPs undergo aging and leaching during sludge dewatering process, and MP leachates may negatively affect the wastewater treatment system.

Carbon footprint analysis of wastewater treatment processes coupled with sludge in situ reduction

The goal of this study was to assess the impacts or benefits of sludge in situ reduction (SIR) within wastewater treatment processes with relation to global warming potential in wastewater treatment plants, with a comprehensive consideration of wastewater and sludge treatment. The anaerobic side-stream reactor (ASSR) and the sludge process reduction activated sludge (SPRAS), two typical SIR technologies, were used to compare the carbon footprint analysis results with the conventional anaerobic - anoxic - oxic (AAO) process. Compared to the AAO, the ASSR with a typical sludge reduction efficiency (SRE) of 30 % increased greenhouse gas (GHG) emissions by 1.1 - 1.7 %, while the SPRAS with a SRE of 74 % reduced GHG emissions by 12.3 - 17.6 %. Electricity consumption (0.025 - 0.027 kg CO2-eq/m3), CO2 emissions (0.016 - 0.059 kg CO2-eq/m3), and N2O emissions (0.009 - 0.023 kg CO2-eq/m3) for the removal of secondary substrates released from sludge decay in the SIR processes were the major contributor to the increased GHG emissions from the wastewater treatment system. By lowering sludge production and the organic matter content in the sludge, the SIR processes significantly decreased the carbon footprints associated with sludge treatment and disposal. The threshold SREs of the ASSR for GHG reduction were 27.7 % and 34.6 % for the advanced dewatering - sanitary landfill and conventional dewatering - drying-incinerating routes, respectively. Overall, the SPRAS process could be considered as a cost-effective and sustainable low-carbon SIR technology for wastewater treatment.

The Impacts of Cellulose on Volatile Fatty Acid Production and the Microbial Community in Anaerobic Fermentation of Sludge at High and Medium Temperatures

During large-scale sewage treatment, a large amount of excessive sludge is produced, which will cause serious pollution in the environment. In recent years, anaerobic digestion technology has been widely promoted because it can achieve better sludge reduction, and the products and byproducts after anaerobic digestion can be fully utilized as resources. In this study, cellulose was added as the co-fermentation substrate during the fermentation process at 30 ℃ and 50 ℃ to enhance the production of VFAs. The result indicated that cellulose could significantly increase the yield of VFAs in both 30 ℃ and 50 ℃. Meanwhile, COD and reducing sugar generation in the fermentation process were also measure. Analysis of the microbial community structure at the class and genus levels revealed that the proportion of several genus closely related with cellulose degradation such as Cellvibrio, Fibrobacter, and Sporocytophaga were significantly increased with the addition of cellulose. Co-fermentation was recognized as an economic and environmental friendly strategy for sludge and other solid waste treatment. The analysis of the effect of cellulose as a substrate on the production of VFAs at high and medium temperatures is highly important for exploring ways to increase the production of VFAs in anaerobic fermentation.

Low-cost optimization of industrial textile landfill sludge re-dewatering using ferrous sulfate and blast furnace slag

This study was based on an industrial sludge landfill with a scale of 1 million cubic meters, which had been filled for more than 10 years. It focused on the secondary dewatering of industrial textile landfill sludge (LS) with a total organic carbon (TOC) content greater than 50% and a volatile suspended solids to suspended solids (VSS/SS) ratio of 0.59. A response surface methodology (RSM) model was established using the coagulant ferrous sulfate (FeSO4) and conditioning agents such as hydrated magnesium oxide (MgO), blast furnace slag (BFS), and calcium oxide (CaO). By solving the RSM equations for the respective indicators, the optimal dosages of FeSO4, MgO, and BFS were determined to be 90 mg/g of dry sludge (DS), and for CaO 174.85 mg/g DS. Further examinations of the dewatering performance, apparent properties, extracellular polymeric substances (EPS) components, rheological characteristics, moisture distribution, and pollutant content variation led to the development of a green waste-based dewatering agent composed of FeSO4 and BFS. In small-scale diaphragm plate and frame filter press tests, the optimal water content (WC) was 69.11%. In the final production-scale experiments, it was 65.72%, with the actual application cost being only 13.07 $/ton DS. Additionally, when FeSO4 and BFS were used together, the combined action of Fe and Si could significantly reduce the biotoxicity of heavy metals (HMs), cut down 75.2% of the LS's TOC, and effectively reduced the leaching of organic substances from the leachate, which was beneficial for subsequent disposal. In conclusion, the combined use of FeSO4 and BFS for the secondary dewatering of industrial textile LS was economically efficient, effective in dewatering, and had significant harm reduction effects, making it a worthwhile for waste treatment.

Deep dewatering of sludge and resource recovery of hydroxyapatite: A recyclable approach via ionic liquid biphasic system and hydrogen bonds reformation

Deep dewatering of Waste Activated Sludge (WAS) through mechanical processes remains inefficient, primarily due to the formation of a stable hydrogen bonding network between the biopolymers and water, which consequently leads to significant water trapped by Extracellular Polymeric Substances (EPS). In this study, a novel and recyclable treatment for WAS based on Ionic Liquids (ILs) was established, named IL-biphasic aqueous system (IL-ABS) treatment. Specifically, the IL-ABS formed in WAS facilitated rapid and efficient in-situ deep dewatering while concurrently recovering hydroxyapatite. The water content decreased from an initial 98.27 % to 65.35 % with IL-ABS, formed by 1-Butyl-3-methylimidazolium bromide (BmimBr) and K3PO4 synthesized from waste H3PO4. Moreover, the recycled BmimBr maintaining the water content of the dewatered sludge consistently between 65.61 % and 67.25 % across five cycles, exhibited remarkable reproducibility. Through three-dimensional excitation-emission matrix, lactate dehydrogenase analyses and confocal laser scanning microscopy, the high concentration of BmimBr in the upper phase effectively disrupted the cells and EPS, which exposed protein and polysaccharide on the EPS surface. Subsequently, the K3PO4 in the lower phase led to an enhanced salting-out effect in WAS. Furthermore, FT-IR analysis revealed that K3PO4 disrupted the original hydrogen bonds between EPS and water. Then, BmimBr formed numerous hydrogen bonds with the sludge flocs, leading to deep dewatering and agglomeration of the sludge flocs during the unique phase separation process of IL-ABS. Notably, sludge-derived hydroxyapatite product exhibited remarkable adsorption capacity for prevalent heavy metal contaminants such as Pb2+, Cd2+ and Cu2+, with efficiencies comparable to those of commercial hydroxyapatite, thereby achieving the resource utilization of waste H3PO4. Moreover, economic calculations demonstrated the suitability of this novel treatment. This innovative treatment exhibits potential for practical applications in the non-mechanical deep dewatering of WAS.

Hydrothermal liquefaction of municipal sludge and its products applications

Hydrothermal liquefaction (HTL) is an effective medium-temperature, high-pressure thermochemical process to dispose municipal sludge (MS), and biocrude (a crude bio-oil) is its main product. Many efforts are continued extensively to improve conversion efficiency and to promote industrial application of this technology. This work focuses on critical influencing factors (e.g., reaction temperature, residence time, atmosphere, solvent, catalyst, and pretreatment) and fundamental transformation mechanisms of main components (i.e., lipids, proteins, and carbohydrates) in MS HTL. It also analyzes migration behavior of heavy metals during MS HTL, which can provide a reference for subsequent recovery of nutrients from HTL products. Moreover, the applications of MS HTL products are systematically expounded, and potential challenges and opportunities are highlighted as well. It is necessary to develop advanced methods of catalyst recovery and innovative biocrude upgrading methods so as to reduce HTL investment and operating costs. Reusing aqueous phase and solid phase products as reaction medium and catalyst carrier separately after MS HTL is feasible to realize resource utilization of MS. This information can provide valuable guidance to promote MS HTL industrialization.

Effect of initial water content on the dewatering performance of freeze-thaw preconditioned landfill sludge

Freeze-thawing (F/T) is an effective method of sludge dewatering preconditioning and has been studied in many studies. However, previous studies have taken landfill sludge from different regions, filled for different length of time or at different depth, resulting in large differences in initial water content and different treatment effects. Therefore, the effect of initial water content on the dewatering characteristics of F/T preconditioned landfill sludge has been investigated. The sludge with different initial water contents was firstly preconditioned by one F/T cycle. Then the F/T sludge was vacuum filtered and compared with the dewatering performance of FeCl3 preconditioned sludge with the same water content. Finally, the mechanism of the initial water content on the effect of F/T preconditioning was analyzed by the change of sludge internal composition. The results show that the higher the initial water content of the sludge, the greater the improvement of its dewatering performance after F/T preconditioning. The specific resistance and water content after filtration of sludge after F/T conditioning decreased greatly with the increase of the initial water content, reaching their respective minimum values of 13.3 × 1012 m/kg and 58.3% at 85% and 87.5%. These values are lower than the optimal values observed for the sludge conditioned by FeCl3. With the rise in initial water content, the driving force at the ice-water interface gains strength. Small particles aggregate into larger flocs, forming stable drainage channels that enhance the dewatering performance of sludge. Once the initial water content surpasses 85%, the squeezing force exerted by ice crystals amplifies the degree of cracking in sludge particles, releasing bound water and further decreasing the water content of sludge.

Application of coffee ground-based skeleton builder with FeCl3 for conditions of pre-dewatered sludge toward further deep dewatering

Skeleton builders are essential for achieving deep sludge dewatering. In this study, a novel spent coffee ground (SCG)-based skeleton builder was developed to attain deep sludge dewatering by combined conditioning with FeCl3, and possible mechanisms were examined. Through different surface analysis techniques, it was demonstrated that at a pyrolysis temperature of 300 °C, the spent coffee ground biochar (SCGB-300) has an intact pore structure, a rigid carbon skeleton, and large oxygen-containing functional groups, making it the best skeleton builder for sludge dewatering. When combined with FeCl3 for conditioning, the structure of SCGB-300 remained intact under high pressure and played important role. The rich porous structure facilitated water drainage. During the sludge conditioning, small amount of positive charge on the surface of SCGB-300 further increased the zeta potential of sludge through charge neutralization. At the same time, the adsorption of SCGB-300 removed viscous hydrophilic substances and further improved the dewatering performance. At an optimum dosage of 6% (dry solid, DS) FeCl3 and 30% SCGB-300 (DS), the moisture content of sludge was reduced from 85.47% to 63.35%, and the dewatering rate was increased from 46.08% to 70.03%. Therefore, SCGB is a promising skeleton builder for sludge conditioning and deep dewatering.

Molecular structure-dependent contribution of reactive species to organic pollutant degradation using nanosheet Bi2Fe4O9 activated peroxymonosulfate

Iron-based catalysts have attracted increasing attention in heterogeneous activation of peroxymonosulfate (PMS). However, the activity of most iron-based heterogenous catalysts is not satisfactory for practical application and the proposed activation mechanisms of PMS by iron-based heterogenous catalyst vary case by case. This study prepared Bi₂Fe₄O₉ (BFO) nanosheet with super high activity toward PMS, which was comparable to its homogeneous counterpart at pH 3.0 and superior to its homogeneous counterpart at pH 7.0. Fe sites, lattice oxygen and oxygen vacancies on BFO surface were believed to be involved in the activation of PMS. By using electron paramagnetic resonance (EPR), radical scavenging tests, ⁵⁷Fe Mössbauer and ¹⁸O isotope-labeling technique, the generation of reactive species including sulfate radicals, hydroxyl radicals, superoxide and Fe (IV) were confirmed in BFO/PMS system. However, the contribution of reactive species to the elimination of organic pollutants very much depends on their molecular structure. The effect of water matrices on the elimination of organic pollutants also hinges on their molecular structure. This study implies that the molecular structure of organic pollutants governs their oxidation mechanism and their fate in iron-based heterogeneous Fenton-like system and further broadens our knowledge on the activation mechanism of PMS by iron-based heterogeneous catalyst.

Simultaneous sludge minimization and membrane fouling mitigation in membrane bioreactors by using a microaerobic - Settling pretreatment module

Membrane fouling is the major obstacle for membrane bioreactors operated at a long sludge retention time to reduce sludge production. In this study, a sludge process reduction (SPR) module, consisting of a microaerobic tank and a settler, was inserted before an anoxic/oxic MBR (AO-MBR) to achieve dual objectives of fouling alleviation and sludge reduction. Three SPR-MBRs were operated to investigate influences of sludge recirculation ratios from the SPR settler to the microaerobic tank on process performance. Compared to AO-MBR, the SPR-MBRs reduced sludge production by 43.1-56.4% by maintaining sludge retention times above 175 d, and decreased foulant layer resistance and pore clogging resistance. Inserting SPR reduced the accumulation of dissolved organic matters and extracellular polymeric substances, enlarged sludge flocs, and decreased sludge viscoelasticity. However, increasing R_SPR stimulated outward diffusion of extracellular polymeric substances and increased sludge viscosity. SPR-MBRs achieved effective sludge reduction by enriching hydrolytic (Trichococcus and Aeromonas) and fermentative genera (Lactococcus, Paludibacter, Macellibacteroides, and Acinetobacter) in the SPR, and alleviated membrane fouling by prohibiting the growth of extracellular polymeric substance-secreting bacteria and enriching filamentous bacteria to enlarge particle size. The results revealed that the SPR-MBR maximized sludge reduction with a very long sludge retention time, and alleviated membrane fouling synchronously.

Anti-clogging mechanism of freeze-thaw combined with step vacuum preloading in treating landfill sludge

Globally, landfill sludge (LS) has accumulated in large quantities, and its reduction and dewatering are urgently needed. To address pollution problems and clogging of drainage boards caused by chemical conditioning combined with traditional vacuum preloading (TVP), a freeze-thaw combined with step vacuum preloading (F/T-SVP) method is proposed. A comparative experimental study was carried out between TVP and SVP to explore the anti-clogging mechanism of F/T-SVP in treating LS. As a result, the water discharge for the original sludge (OS) is 1840 ml, the water discharge for TVP is 8830 ml and for SVP is 10,010 ml; The total settlement of SVP is 16% higher than that of TVP; TVP has a volume reduction ratio of 57.6%, while SVP has 66.8%; the OS's water content was 86%, which was reduced to 57.6% by F/T-SVP; The center of the drainage board of TVP is seriously clogged, while the particles of SVP are evenly distributed; The tendency for small particles to undergo transport is relatively low at the beginning of SVP, which can effectively reduce clogging; TVP mainly focuses on the compression of large pores into small pores, and SVP mainly focuses on the compression of large into small pores and micropores. In SVP, there is more consolidation and a more compact structure. When F/T-SVP is used to treat LS, the pores are gradually penetrated, effectively avoiding the generation of clogging and improving LS's drainage and consolidation.

An all-organic conditioning method to achieve deep dewatering of waste activated sludge and the underlying mechanism

Among the common treatment/disposal routes of excessive activated sludge from municipal wastewater treatment plant, dewatering process functions as an essential pre-/post-treatment for volume minimization and transportation facilitation. Since inorganic coagulants have long been criticized for their high dosage and solid residue in sludge cake, there is an urgent need for investigations regarding the potential of applying organic chemicals as the conditioner. In this study, combined use of poly dimethyldiallylammonium chloride (PDMD) and tannic acid (TA) were investigated as an all-organic co-conditioning method for sewage sludge pre-treatment. Results showed that this all-organic conditioning strategy can effectively improve the dewaterability of sewage sludge. The capillary suction time reduced from 128.8 s to 23.1 s, and the filtration resistance reduced from 1.24 × 10¹² cm/g to 7.38 × 10¹⁰ cm/g. The moisture content of dewatered sludge cake decreased to as low as 55.83%, showing the highest dewatering efficiency reported so far. In addition, the combination of PDMD and TA maximized the treating efficiency with very limited consumption of conditioners (added up to 4% of total solid). Based on the physic-chemical and rheological property investigation, it was proposed that the intermediate molecular weight polymer-based flocculation process and the TA agent-based protein precipitation process, could remarkably strengthen the compactness and structure robustness of sludge. In all, this PDMD-TA-based conditioning method suggested practical significance in consideration of its cost-effectiveness and disposal convenience of sludge cake.

Enhancing the Dewaterability of Oily Cold Rolling Mill Sludge Using Quicklime as a Conditioning Agent

The high moisture and viscosity characteristics of oily cold rolling mill (CRM) sludge led to limitations in its recycling. In this paper, the results of using quicklime as a conditioning agent to improve the dewaterability of the oily CRM sludge cake were reported. Quicklime was selected as the best conditioning agent through conditioning-dewatering experiments because it could effectively reduce the viscosity of oily CRM sludge and improve the dewaterability of the oil sludge filter cake. The optimal conditioning effect was obtained when the quicklime dosage was 10%, the temperature was 60 °C, the liquid/solid ratio was 1.5:1, and the time was 30 min. The reduction of specific resistance to filtration was 95.9%, and the coefficient of compressibility of the filter cake decreased from 1.11 to 0.89. The dewatering rate increased from 9.0 to 52.6%. The oily CRM sludge cake formed an incompressible rigid porous structure because of conditioning with quicklime. In addition, after conditioning with quicklime, the oil content, chemical oxygen demand, and turbidity of the filtrate decreased, and the composition of the dried filter cake met the requirements of ironmaking raw materials. Using quicklime to condition the oily CRM sludge provided a green approach to waste recovery and sustainable management.

Triple action of FeCl3-assisted hydrothermal treatment of digested sludge for deep dewatering

In this study, a FeCl₃-assisted hydrothermal treatment (HTT) process under mild conditions (90 °C-130 °C) was developed for deep dewatering of anaerobically digested sludge. HTT of sludge at 90 °C-130 °C with 4%-6% Fe³⁺ ions loading based on total sludge solids followed by mechanical dewatering reduced sludge water content from 82% to 38%-53% and sludge weight by 62%-72%. The treatment increased the flowability of sludge through reduction of apparent viscosity and disintegration of colloidal forces between sludge particles. This study unveiled that FeCl₃-assisted HTT process had three mechanisms for improving sludge dewaterability and flowability. The treatment hydrolysed sludge flocs in the presence of Lewis acid FeCl₃ and high temperature (90-130 °C). Fe³⁺ ions also improved dewaterability through the formation of double electric layers and neutralisation of surface negative charges, leading to flocculation of sludge flocs. More importantly, the hydrolysed sludge components produced during HTT process acted as reducing agents and led to in-situ generation of iron oxyhydroxide nanoparticles through reduction-oxidation reactions, further enhancing flocculation/co-precipitation of sludge flocs. The treatment reduced EPS content and changed conformational structures of EPS proteins by breaking down hydrogen bond-maintaining α-helix which led to a loose EPS protein structure and enhanced hydrophobicity and flocculability. Furthermore, the FeCl₃-assisted treatment promoted immobilisation of the majority of heavy metals in the sludge matrix through co-precipitation/complexation reactions with iron species and organic/inorganic matters. This indicates that the FeCl₃-assisted treatment reduced direct toxicity/bioavailability of the majority of heavy metals and the treated sludge may be suitable for land application. Overall, this study provides new insights into mechanism of FeCl₃-assisted HTT process for dewaterability of anaerobically digested sludge and immobilisation of heavy metals.

Effect of freezing temperature on dewatering, consolidation properties, and microstructure of landfill sludge

There is currently a considerable amount of landfill sludge (LS) accumulating worldwide, threatening the surrounding environment and land safety. It is urgent to dewater and reduce LS. Chemical conditioning is the most common treatment for LS, which can pollute the environment and limit resource exploitation. Therefore, a more environmentally friendly and efficient freeze-thaw combined vacuum preloading method is proposed. Experimental studies were conducted to investigate the influence of different freezing temperatures on the dewatering properties, compression and consolidation features, and the mechanism of microstructural change in LS. The results show that the freezing temperature has an important influence on the dewatering, compression, vacuum drainage, consolidation, and microstructure characteristics of LS. The compressibility of LS does not improve when the freezing rate is too high. Freeze-thaw cycles can improve sludge's permeability and consolidation properties by one to two orders of magnitude, and a suitable freezing temperature can significantly improve sludge's permeability and consolidation properties. After vacuum drainage and consolidation, the maximum volume reduction ratio of sludge can reach 55.3%, and the water content of LS can be reduced from 86% to 66%. The distribution of large pores and mesopores in LS increases as the freezing temperature decreases, reaching a maximum of -15 °C, which can substantially improve permeability, drainage, and consolidation efficiency.

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.

Dehydration effect of freeze-thaw on sludge: Temperature spatio-temporal distribution and multi-scale evaluation

The freeze-thaw vacuum method for conditioning pretreated sludge has been proved that it not only has greater dewatering efficiency but also is more ecologically friendly. In this paper, the experiment is improved to address shortcomings in previous freeze-thaw vacuum approach for sludge treatment. The spatio-temporal distribution relationship of distance-time-temperature is developed and divided into two stages by numerically fitting the temperature change of freezing tubes in the sludge. It is expected to guide the time control of large-scale frozen sludge in practical engineering applications to achieve optimal dewatering treatment. Furthermore, the performance of dehydration after the model test is evaluated on multi-scale: settlement and mechanical properties (macroscopic perspective), mean particle size (mesoscopic perspective), and SEM microstructure (microscopic perspective). The results reveal that the improved sludge treatment method of alternating freeze-thaw vacuum procedures, using both prefabricated horizontal drains (PHDs) and prefabricated vertical drains (PVDs), substantially benefits the sludge dewatering and reduction. This method results in an unparalleled volume reduction of 63.51% and a water content reduction to 58.54%. Moreover, in-situ vane shearing strength of the sludge obtained from the improved test meets the strength requirement for the landfill final cover soil, demonstrating that the method is superior in improving mechanical properties.

Enhanced technology for sewage sludge advanced dewatering from an engineering practice perspective: A review

The increasing production of sludge poses significant environmental risks. Sludge disposal and transport are costly because of the high water content (WC). Reducing the WC of sludge is the most efficient strategy to decrease treatment costs. However, the sludge contains a large amount of hydrophilic organic matter, causing poor dewaterability. Therefore, research on preconditioning and mechanical dewatering has great significance for advanced sludge dewatering. In this study, the features of sludge, the advantages and disadvantages of preconditioning methods, and the action mechanisms (including physical, chemical, and biological preconditioning) are thoroughly described. In addition, the dewatering principle and engineering applications of mechanical dewatering techniques are introduced in this manuscript, especially the application of vacuum preloading as an in-situ dewatering technology in sludge. Finally, cost analysis of different conditioning and mechanical dewatering methods is conducted to explore their application feasibility. This manuscript provides new insights for engineering applications of preconditioning methods and mechanical dewatering technology.

Influence of alternating electric field on deep dewatering of municipal sludge and changes of extracellular polymeric substance during dewatering

A self-prepared experimental device made of plexiglass with alternating power supply system was used to study the deep dewatering of municipal dewatered sludge. Considering the reduction rate of sludge water content (W_r) as the index, factors affecting enhanced electric settlement of sludge such as exchange electrode method, voltage gradient, sludge thickness, and mechanical pressure were studied, and the dewatering mechanism was elucidated. The single-factor experiment combined with the surface response method based on the Box-Behnken central experimental design was performed. With W_r as the response value, the voltage gradient conditions, time ratio, and sludge thickness were optimized. Pearson correlation analysis showed that the reduction of proteins/polysaccharides was beneficial to improving the sludge dewatering effect. Tightly bound extracellular polymeric substances (TB-EPSs) showed a significant influence on the sludge dewatering effect. Under the action of the external electric field, particles with negative charge moved toward the anode sludge, water with partial positive charge flowed to the cathode, and the sludge cellular structure was damaged. This resulted in the dissolution of a large number of EPSs and the release of bound water. The anode sludge cake got thickened due to the accumulation of the sludge particles, leading to the increase in resistance. The TB-EPS was deconstructed by the ohmic heating to improve the sludge dewatering effect and achieve deep dewatering. Scanning electron microscopy results showed that the drying problem of anode sludge was alleviated during the dewatering process.

Insights into the dewatering of excavated landfill sludge conditioned by polyferric silicate sulfate

Large quantities of landfill sludge (LS) with higher water content (WC) were stored underground, and excavation and re-dewatering of LS is a sustainable and economic strategy to save landfill space and reduce the leaching of contaminants. In this study, polyferric silicate sulfate (PFSS) was first applied in the conditioning of excavated LS, and the effects of the Si/Fe mass ratio and PFSS dosage on physicochemical properties, dewaterability and rheological properties were investigated. At the best Si/Fe of 0.18, PFSS conditioning obtained compact aggregates with the strongest internal structure, thus achieving the lowest WC. Large sludge flocs were formed, and slime and loosely-bound extracellular polymeric substances were effectively removed with the PFSS dosage above 100 mg/g dried solids, which made the WC to be lower than 51.4%. The whole mechanical compression process of conditioned LS can be described by the modified Terzaghi-Voigt model, and increasing the PFSS dosage induced the release of bound water and migration of the consolidation stage from ternary to secondary. PFSS is an economically sustainable conditioner for LS, integrating multiple functions such as charge neutralization, particle aggregation, interparticle bridging and skeleton building in one chemical.

Insights into vacuum preloading consolidation of landfill sludge based on Fe2+-activated sodium persulfate

To further reduce the in-situ sludge from landfill, Fe²⁺-activated sodium persulfate combined with vacuum preloading was first proposed. Firstly, the effects of optimal Na₂S₂O₈ dosage on the landfill sludge (LS) were investigated by the vacuum filtration experiments. Then, vacuum preloading experiments were conducted on the sludge with different Na₂S₂O₈ dosages to study the water content, water discharge, and settlement. Besides, sludge particle size diversification was carried out by particle size distribution and scanning electron microscopy (SEM) tests. The results were summarized as follows: the specific resistance of filtration (SRF) of LS could be reduced by 98.5% mostly when the Na₂S₂O₈ dosage was 30%; the particle size became significantly smaller, and large particles were converted to small particles; the water content dropped from 86.9 to 58.3%; and the SEM test manifested the oxidation of sodium persulfate caused the destruction of the glial structure of the sludge and the recombination of partial particles.

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.

Adaptability of organic matter and solid content to Fe2+/persulfate and skeleton builder conditioner for waste activated sludge dewatering

Sludge conditioning is important for improved dewatering, with the sludge characteristics impacting the effect of conditioning. A composite conditioner, Fe2+-activated sodium persulfate (Fe2+/SPS) combined with phosphogypsum (PG), was used to examine its impact on sludges with different organic contents (34.6-43.8%) or different solid contents (2.8-5.9%). Response surface optimization analysis shows that when the best conditioning is achieved, the reduction of the specific resistance to filtration (SRF) is not sensitive to organic matter content, but the dewatering performance of the sludge is greatly affected by the solid content. The oxidation role of Fe2+/SPS and the skeleton builder role of PG together affect the conditioning, oxidation playing a major role in conditioning, especially for greater organic matter content. The organic content (maximum ηSOL value was 0.32) also affects the effectiveness of the skeleton builder more than the solid content (Maximum ηSOL value was 0.25). Changes in PG significantly impacts the optimal molar ratio and dosage of Fe2+/SPS. Sludge with greater solid content requires greater Fe2+/SPS dosage to provide stronger oxidation to destroy flocs, and the maximum Fe2+:SPS molar ratio was 1.14 with solid content of 5.9 wt%. The composite conditioning decreases the content of extracellular polymeric substances and proteins/polysaccharides. This study provides new insight into the relationship between the oxidation role of Fe2+/SPS and the skeleton builder role of PG for sludge conditioning strategies according to the optimal conditions.

Elucidating the intensifying effect of introducing influent to an anaerobic side-stream reactor on sludge reduction of the coupled membrane bioreactors

Three anoxic/oxic membrane bioreactors (AO-MBRs) coupled with the anaerobic side-stream reactor (ASSR) with different influent flow distribution ratios (IFDRs) were assessed to elucidate how IFDR in the ASSR affected pollutants removal, sludge reduction, membrane fouling, and potential co-occurrence network of microorganisms. When the IFDR in the ASSR was increased from 0% (ASSR₀-MBR), to 25% (ASSR₂₅-MBR) and 75% (ASSR₇₅-MBR), chemical oxygen demand removal was enhanced and nutrient removal was comparable. Compared to ASSR₀-MBR, ASSR₂₅- and ASSR₇₅-MBR further improved the sludge reduction by 7.6% and 10.9%, respectively. ASSR₂₅-MBR followed cake-complete model due to the weak membrane surface scouring and high concentration of extracellular polymeric substances, while ASSR₀- and ASSR₇₅-MBR fitted cake-standard model. The increased IFDR in the ASSR boosted the relative abundance of hydrolytic and slow-growing bacteria. The co-occurrence networks of sludge reduction, nutrient removal and membrane fouling propensity indicated that the symbiotic relationships were dominant.

Novel insights into enhanced dewaterability and consolidation characteristics of landfill sludge and fresh sludge conditioned by Fe2+ activated sodium persulfate

Considering the reduction and resource utilization of landfill sludge (LS) and fresh sludge (FS), Fe²⁺ activated Na₂S₂O₈ is proposed. The effects of the molar ratio of Fe²⁺/S₂O₈^2- and the addition of Na₂S₂O₈ on the dewatering performance of sludge were studied by vacuum filtration experiments. Consolidation tests were conducted on the sludge with different Na₂S₂O₈ dosage, and the compression, consolidation, and permeability characteristics of the sludge were researched. Besides, via particle size distribution (PSD) and scanning electron microscope (SEM) test, the variation of particle size of sludge was studied from the microscopic perspective. The results are as follows: the specific resistance of filtration (SRF) of LS and FS decreases by 99.3%, 95.2% at an optimal dosage (the molar ratio of (Fe²⁺/S₂O₈^2-) = 1, 30% Na₂S₂O₈); the particle size of LS and FS is significantly smaller; the consolidation and permeability coefficients are increased by 1-2 orders of magnitude compared with non-conditioned sludge; the water content of LS and FS drops from 86.5% to 58.4%, 82.4%-59.7%. The research results have certain guiding significance for the in-situ treatment of sludge deep dewatering.

New methods for quantification of Fenton's reagent addition based on aged sludge indicators to improve filterability

Fenton oxidation can effectively improve the dewaterability of aged sludge. Quantification of the addition of optimal reagents is central to the conditioning and dewatering of aged sludge. Improving the accuracy of quantification is significant to promote cost effectiveness. The effects of reagent addition and the mechanism governing the improved filterability of the aged sludge need to be understood uniformly. In this study, the optimal reagent additions have been determined using the response surface method (RSM) for five out of the eight aged sludges that were investigated. The physicochemical characteristics of eight aged sludges, including the extracellular polymer substance, undissolved organic matter, and suspension structure network, were investigated. Meanwhile, a comprehensive correlation analysis of critical indicators was conducted to investigate the interactions among the properties of the aged sludge. The effects of these interactions on the conditioning and filtration processes were examined, and a unified understanding of the combination of factors affecting the optimal reagent addition was obtained. The key factors were aggregate size, dewatering extent, yield stress, and organic substance content. Based on these results, a new reagent addition quantification method was developed along with an empirical model of the relationship between physicochemical properties and the economically optimal reagent addition.

Effects of freeze-thaw and chemical preconditioning on the consolidation properties and microstructure of landfill sludge

At present, a large amount of landfill sludge(LS) has been accumulated all over the world. For environmental and engineering purposes, there is an urgent need for deep dewatering and volume reduction of LS. The deep dewatering of LS mainly uses the method of chemical preconditioning and mechanical dewatering, which is easy to cause environmental pollution and is not conducive to the subsequent resource treatment of LS. To find a more environmentally friendly and efficient method for deep dewatering of LS, an in-situ treatment method combining freeze-thaw and vacuum preloading was proposed. In this paper, based on the existing research, through compression consolidation test and MIP, SEM micro test, the consolidation properties and microstructure of LS after freeze-thaw and chemical preconditioning were studied, and the vacuum consolidation principle of different preconditioning was explored. The results show that: Both FeCl₃ and freeze-thaw preconditioning can increase the permeability coefficient and consolidation coefficient by one to two orders of magnitude; After freeze-thaw preconditioning, the void ratio of sludge decreases and the permeability coefficient increases; Under low consolidation pressure, the mechanical properties of the two kinds of pretreated sludge changed significantly; The original sludge is mainly composed of small pores. After FeCl₃ conditioning, the large pores and mesopores increased significantly, while the small pores decreased. After freeze-thaw, the large pores and mesopores increase greatly, while the small pores decrease greatly; The original sludge is in the form of a dispersive flocculent structure with many impurities. After freeze-thaw, the intercluster pores increase, showing a honeycomb structure. After FeCl₃ conditioning, the sludge structure is more compact and uniform. The change of microstructure and consolidation characteristics of sludge after conditioning reflects the difference of two different preconditioning mechanisms.

In-situ sludge reduction performance and mechanism in an anoxic/aerobic process coupled with alternating aerobic/anaerobic side-stream reactor

Activated sludge process with anaerobic side-stream reactors (SR) in the sludge recirculation can achieve in-situ sludge reduction, but sludge reduction efficiency is limited with the low hydraulic retention time (HRT) of SR. An anoxic/aerobic (AO) process, AO coupled with anaerobic SR and AO coupled with alternating aerobic/anaerobic side-stream reactor (AO-OASR) were operated to investigate enhancing effects of alternative aerobic and anaerobic condition (AltOA) in SR on sludge reduction and pollutants removal performance. The AltOA was firstly proposed into SR with a low HRT during the long-term continuous operation. The results showed that AO-OASR presented a lower effluent COD concentration (29.6%) with no adverse effect on nitrogen removal, compared to AO, owing to the intensified refractory carbon reuse in the mainstream aerobic tank. The sludge yield in AO-OASR (0.240 g SS/g COD) was 39.7% lower than that in AO. The OASR accelerated sludge lysis and particle organic matter hydrolysis due to the weakened network strength of flocs, leading to an enhanced increase (17.3 mg/L) of dissolved organic matter (DOM), especially for the fraction of molecular weight (MW) < 25 kDa. The OASR reduced the adenosine triphosphate (ATP) content for heterotrophic anabolism in the mainstream reactor by 42.9%, compared to the ASR. MW < 25 kDa of DOM caused the disturbance of oxidative phosphorylation with a decreasing ATP synthase activity under high-level electronic transport system, leading to ATP dissipation. The cooperation interaction of predator (norank_Chitinophagales), hydrolytic/fermentative bacteria (unclassified_Bacteroidia and Delftia), and slow grower (Trichococcus) played a key role in improving the sludge reduction and carbon reuse in AO-OASR. The results provided an efficient and cost-saving technology for sludge reduction with modified SR under low HRT, which is meaningful to overcome the present bottleneck of deficient reduction efficiency for application in wastewater treatment plants.

Enhancement of sludge dewaterability by a magnetic field combined with coagulation/flocculation: a comparative study on municipal and citric acid-processing waste-activated sludge

The difficulties in dewatering waste-activated sludge (WAS) using mechanical devices have caused great problems in sludge transportation and disposal. Herein, coagulation and flocculation are combined with the use of a magnetic field as a clean and low-energy physical treatment method to enhance the dewaterability of municipal and citric acid-processing WAS. It is shown that the use of the magnetic field had a significant effect on the capillary suction time (CST) of municipal WAS but not on the specific resistance filtration (SRF) and CST of the citric acid WAS. The differences in the magnetic field effects were due to differences in the sludge properties. For municipal WAS, the particle size decreased, the zeta potential remained unchanged, and the viscosity decreased, whereas in the citric acid WAS, the particle size increased, the absolute value of the zeta potential decreased, and the viscosity increased. In addition, these effects were also confirmed with studies of the water state and micro-morphology analyses. It is shown that the acidification of the municipal WAS and coagulation of citric acid WAS were likely the reasons for the enhancement of their dewaterability, respectively. This study confirmed that the use of a magnetic field combined with coagulation/flocculation may serve as an effective sludge conditioning method; however, the treatment conditions may vary with the sludge type.

Revealing the heating value characteristics of sludge-based hydrochar in hydrothermal process: from perspective of hydrolysate

Hydrothermal treatment (HT) is a promising method to convert sewage sludge to hydrochar biofuel. The heating value is directly correlated to the carbon content in hydrochar; however, the release of organic matter from sludge to hydrolysate and the transfer of the Maillard reaction products generated in the hydrolysate to the solid phase alter the carbon content in hydrochar. In this study, the relationship between hydrolysate and heating value of sludge-based hydrochar was presented, aiming to explain how the calorific value of hydrochar was affected by HT conditions. We adopted a direct combustion test to verify its clean combustion features. Hydrochar derived at 260 °C and residence time of 4 h (HC 260-4) exhibited the highest calorific value (HHV_daf = 26.23 MJ/kg) with an energy density of 1.43, and its fuel characteristics were similar to those of lignite. The increase in the HT temperature and residence time up to 260 °C and 4 h, respectively, was beneficial for enhancing HHV_daf. Conversely, further increase of the HT temperature to 300 °C and HT time to 6 h yielded a decrease in HHV_daf. Investigation of the underlying mechanism revealed that the protein and polysaccharide releasing from sludge to hydrolysate occurred the Maillard reaction (MR). The generated humic-like Maillard reaction product (MRP) was transferred to hydrochar, inducing an increase in the carbon content and calorific value and a decrease in the organic content of hydrolysate. As the carbohydrate content in the hydrolysate decreased, the MR was terminated, so no more MRP was transferred to hydrochar. At the same time, the protein was still continuously released at higher temperatures and longer residence times, yielding a decline in the HHV_daf. Moreover, clean energy utilization was verified from the reduced nitrogen content in hydrochar and lower CO and NO_x emission of HC 260-4 in the combustion test. After the HT, increased hydrophobicity and a lower fraction of bound water improved the dewaterability, which is of great significance for applying hydrochar as biofuel. The findings of this study provided a new perspective to explain the heating value generation of hydrochar and more direct evidence to assess its clean combustion properties, with promising perspectives for practical applications.

Changes in the characteristics of dissolved organic matter during sludge treatment: A critical review

Dissolved organic matter (DOM) of sludge is a heterogeneous mixture of high to low molecular weight organic substances which is including proteinaceous compounds, carbohydrates, humic substances, lipids, lignins, organic acids, organic micropollutants and other biological derived substances generated during wastewater treatment. This paper reviews definition, composition, quantification, and transformation of DOM during different sludge treatments, and the complex interplay of DOM with microbial communities. In anaerobic digestion, anaerobic digestion-refractory organic matter, particularly compounds showing polycyclic steroid-like, alkane and aromatic structures can be generated after pretreatment. During dewatering, the DOM fraction of low molecular weight proteins (< 20,000 Dalton) is the key parameter deteriorating sludge dewaterability. During composting, decomposition and polymerization of DOM occur, followed by the formation of humic substances. During landfill treatment, the composition of DOM, particularly humic substances, are related with leachate quality. Finally, suggestions are proposed for a better understanding of the transformation and degradation of DOM during sludge treatment. Future work in sludge studies needs the establishment and implementation of definitions for sample handling and the standardization of DOM methods for analysis, including sample preparation and fractionation, and data integration. A more detailed knowledge of DOM in sludge facilitates the operation and optimization of sludge treatment technologies.

Experimental study on treating landfill sludge by preconditioning combined with vacuum preloading: Effects of freeze-thaw and FeCl3 preconditioning

The deep dewatering of landfill sludge (LS) mainly uses the methods of chemical preconditioning and mechanical dewatering, which is easy to cause environmental pollution and is not conducive to the subsequent recycling treatment of sludge. To find a more environment-friendly and efficient method for LS's deep dewatering and volume reduction, an in-situ sludge treatment method combining freeze-thaw (F/T) preconditioning and vacuum preloading was proposed. Firstly, the F/T test of LS was carried out to explore the optimum freezing temperature. FeCl₃, the most widely used agent, was selected as the chemical preconditioning. Then carry out vacuum preloading model box test. The data were compared after the test. The mechanisms of the two different sludge preconditioning methods on the LS's consolidation were analyzed. The results show that: after freezing, the specific resistance of LS decreases obviously, the overall particle size increases, the content of small particles decreases. Too fast freezing rate is not conducive to the LS's dewatering. After preconditioning (F/T and FeCl₃) combined with vacuum preloading, the volume reduction ratio was 57.1% and 41.1% respectively, the water content was reduced from 73.4% to 53.7% and 58.1%, and the unconfined compressive strength(UCS) was improved from 15.5 kPa to 50.9 kPa and 77.3 kPa. The total water discharge, drainage rate, volume reduction, and water content of freeze-thaw preconditioned LS are better than FeCl₃ preconditioned, while FeCl₃ preconditioned LS has higher UCS. F/T can aggregate small sludge particles but the acidification and hydrolysis of FeCl₃ always produce small particle, which is not conducive to the consolidation of LS during vacuum preloading.

Applying organic polymer flocculants in conditioning and advanced dewatering of landfill sludge as a substitution of ferric trichloride and lime: Mechanism, optimization and pilot-scale study

In this study, poly dimethyl diallyl ammonium chloride (PDADMAC) and polyacrylamide (PAM) were applied to substitute ferric trichloride (FeCl₃) and lime conditioning for advanced dewatering of landfill sludge (LS). Four response surface methodology (RSM) models were constructed for FeCl₃-lime, FeCl₃-PAM, PDADMAC-lime and PDADMAC-PAM, and identical dosages, namely 29.86, 57.91, 5.73 and 2.99 mg/g dry solids (DS) for FeCl₃, lime, PDADMAC and PAM, were obtained by solving the system of four RSM equations at water content of 60% to investigate conditioning mechanisms. Compared to FeCl₃-lime, PDADMAC-PAM conditioning had strong charge neutralization and bridging performance, and obtained conditioned LS with large flocs size, strong network structure and rapid dewatering rate. By integrating RSM with nonlinear programming for optimization, the total cost of PDADMAC-PAM route was saved by 7.9% and close to FeCl₃-lime, and the optimized condition with dosages of 1.93 and 3.47 kg/t DS was further confirmed by pilot-scale experiments. The results indicated that PDADMAC-PAM was a feasible substitute for FeCl₃-lime in sludge conditioning, and showed more advantage if dewatered sludge was further treated by incineration.

Investigation on the variations of sludge water holding capacity of electro-dewatering process

Since the effect of electro-dewatering (EDW) on sludge water holding capacity was unknown, tests were conducted in this study to investigate the water holding capacity of EDW sludge and the potential mechanism related to the sludge physicochemical characteristics, EPS properties and sludge structure. Sludge was dewatered to the average moisture content (AMC) of 80%, 70% and 60% with different applied voltages at 20, 30 and 40V in EDW, respectively. Then the dewatered sludge near the anode and cathode were rewatered. The variation of sludge water holding capacity in EDW process was evaluated in terms of filterability and saturated moisture content (SMC), and the filterability was assessed by the specific resistance to filtration (SRF) of rewatered sludge. The results indicated that SRF of rewatered sludge near the cathode increased greatly. The proteins/polysaccharides (PN/PS) of loosely bound extracellular polymeric substances (LB-EPS) was significantly positively correlated with SRF (r = 0.891, p < 0.01). Moreover, the exposure of hydrophobic sites or groups in PN near the cathode improved the surface hydrophobicity of sludge, which reduced the filterability. In addition, higher voltage could destroy the sludge structure near the anode at the later stage of EDW process, leading to the decrease of SRF and SMC. These results expanded the knowledge about changes in sludge properties and water holding capacity during EDW process.

Insight into conditioning landfill sludge with ferric chloride and a Fenton reagent: Effects on the consolidation properties and advanced dewatering

The landfill sludge in storage reservoirs needs to be dewatered and disposed of for environmental and engineering purposes. The key factors are the high organic matter content and low permeability. Chemical conditioning is considered an efficient method for adjusting the properties of sludge. In this paper, two typical chemical agents, FeCl₃ and a Fenton reagent with different additive amounts, are studied and compared for dewatering and consolidation purposes. Compression experiments and consolidation experiments are compared, and the coefficient of compressibility and compression index are obtained and compared. Then, the sludge permeability, grain size distribution variations, specific resistance to filtration (SRF) and morphology observations are considered to analyse the treatment mechanism. The results indicate that the properties of landfill sludge will change as the curing time increases. FeCl₃ and Fenton are both effective in improving the consolidation and permeability properties of sludge. For the conditioning process, the optimum FeCl₃ content is 20%, and the process is dominated by coagulation if FeCl₃ is less than 20%; otherwise, it is dominated by hydrolysis. For the Fenton reagent, the optimum Fe²⁺ content and H₂O₂ content are 4% and 12%, respectively. The depolymerization effect of the Fenton reagent leads to the oxidation and recombination of the polar group on extracellular polymeric substances (EPSs). The results can be used to explain the conditioning mechanism of the effective agents of FeCl₃ and Fenton and compare the corresponding consolidation properties. The consolidation characteristics provide a reference for further application of vacuum preloading in the sludge disposal process.

Biological nutrient removal in the anaerobic side-stream reactor coupled membrane bioreactors for sludge reduction

An anoxic-aerobic membrane bioreactor (AO-MBR), an anaerobic side-stream reactor (ASSR) coupled MBR (A-MBR), and an MBR with ASSR packed with carriers (AP-MBR) were operated parallelly to investigate biological nutrient removal, microbial community structure and mass balance of nutrients in sludge reduction systems. Compared to AO-MBR, A-MBR and AP-MBR were both efficient in COD and NH₄⁺-N removal, had significantly higher nitrogen removal, reduced sludge production by 35.0% and 45.9%, but deteriorated biological phosphorus removal. Nitrosomonadaceae and Nitrospira were major bacteria responsible for ammonium and nitrite oxidation in the three systems. Inserting ASSR and packing carriers both favored denitrifying bacteria enrichment and organic substances release, and thus resulted in higher nitrate uptake rate (NUR) in the anoxic tank. Higher endogenous NUR in ASSR than in anoxic tank also indicated that ASSR and carriers both accelerated sludge decay. Denitrification and sludge reduction occurred in ASSR played important roles in biological nutrient removal.