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

Tannic Acid Modulation of Substrate Utilization, Microbial Community, and Metabolic Traits in Sludge Anaerobic Fermentation for Volatile Fatty Acid Promotion

Anaerobic fermentation is a crucial route to realize effective waste activated sludge (WAS) resource recovery and utilization, while the overall efficiency is commonly restrained by undesirable disruptors (i.e., chemical dewatering agents). This work unveiled the unexpectedly positive effects of biodewatering tannic acid (TA) on the volatile fatty acids (VFAs) biosynthesis during WAS anaerobic fermentation. The total VFAs yield was remarkably increased by 15.6 folds with enriched acetate and butyrate in TA-occurred systems. TA was capable to disintegrate extracellular polymeric substances to promote the overall organics release. However, TA further modulated the soluble proteins structure by hydrogen bonding and hydrophobic interactions, resulting in the decrease of proteins bioavailability and consequential alteration of metabolic substrate feature. These changes reshaped the microbial community and stimulated adaptive regulatory systems in hydrolytic-acidogenic bacteria. The keystone species for carbohydrate metabolism (i.e., Solobacterium and Erysipelotrichaceae) were preferentially enriched. Also, the typical quorum sensing (i.e., enhancing substrate transport) and two-component systems (i.e., sustaining high metabolic activity) were activated to promote the microbial networks connectivity and ecological cooperative behaviors in response to TA stress. Additionally, the metabolic functions responsible for carbohydrate hydrolysis, transmembrane transport, and intracellular metabolism as well as VFA biosynthesis showed increased relative abundance, which maintained high microbial activities for VFAs biosynthesis. This study underscored the advantages of biodewatering TA for WAS treatment in the context of resource recovery and deciphered the interactive mechanisms.

Physical conditioning methods for sludge deep dewatering: A critical review

Sludge is an inevitable waste product of sewage treatment with a high water content and large volume, it poses a significant threat of secondary pollution to both water and the atmosphere without proper disposal. In this regard, dewatering has emerged as an attractive method in sludge treatment, as it can reduce the sludge volume, enhance its transportability and calorific value, and even decrease the production of landfill leachate. In recent years, physical conditioning methods including non-chemical conditioners or energy input alone, have been extensively researched for their potential to enhance sludge dewatering efficiency, such as thermal treatment, freeze-thaw, microwave, ultrasonic, skeleton builders addition, and electro-dewatering, as well as combined methods. The main objective of this paper is to comprehensively evaluate the dewatering capacity of various physical conditioning methods, and identify key factors affecting sludge dewatering efficiency. In addition, future research anticipated directions and outlooks are proposed. This work is expected to provide valuable insights for developing efficient, eco-friendly, and low-energy consumption techniques for deep sludge dewatering.

Enzyme-enhanced acidogenic fermentation of waste activated sludge: Insights from sludge structure, interfaces, and functional microflora

Anaerobic fermentation is widely installed to recovery valuable resources and energy as CH4 from waste activated sludge (WAS), and its implementation in developing countries is largely restricted by the slow hydrolysis, poor efficiency, and complicate inert components therein. In this study, enzyme-enhanced fermentation was conducted to improve sludge solubilization from 283 to 7728 mg COD/L and to enhance volatile fatty acids (VFAs) yield by 58.6 % as compared to the conventional fermentation. The rapid release of organic carbon species, especially for tryptophan- and tyrosine-like compounds, to outer layer of extracellular polymeric substance (EPS) occurred to reduce the structural complexity and improve the sludge biodegradability towards VFAs production. Besides, upon enzymatic pretreatment the simultaneous exposure of hydrophilic and hydrophobic groups on sludge surfaces increased the interfacial hydrophilicity. By quantitative analysis via interfacial thermodynamics and XDLVO theory, it was confirmed that the stronger hydrophilic repulsion and energy barriers in particle interface enhanced interfacial mass transfer and reactions involved in acidogenic fermentation. Meanwhile, these effects stimulate the fermentation functional microflora and predominant microorganism, and the enrichment of the hydrolytic and acid-producing bacteria in metaphase and the proliferation of acetogenic bacteria, e.g., Rubrivivax (+9.4 %), in anaphase also benefits VFAs formation. This study is practically valuable to recovery valuable VFAs as carbon sources and platform chemicals from WAS and agriculture wastes.

Sludge biodrying coupled with photocatalysis improves the degradation of extracellular polymeric substances

The efficiency of sludge dewatering is limited by extracellular polymeric substances (EPS) during biodrying. This study investigated the effect of photocatalysis-mediated EPS degradation on sludge dewatering performance during the sludge biodrying process. The photocatalysis of municipal sludge was first carried out to choose a cost-efficient catalyst. Then sludge biodrying tests were performed using TiO2-coated amendment (TCA) and uncoated amendment (TUCA) as the control. Municipal sludge photocatalysis results showed that using TiO2 could efficiently degrade carbohydrates and proteins in the EPS within 60 min. After 20-day biodrying, photocatalysis significantly promoted a reduction in the moisture content and EPS by 17.64% and 6.88%, respectively. The surface-enhanced Raman scattering (SERS) intensities of the C-C-O symmetric stretching vibration peak of D-lactose and the C-S stretching vibration peak of cysteine were significantly decreased by approximately 33.19% and 44.76%, respectively, indicating that photocatalysis indeed promoted the reduction of polysaccharides and cysteine in the EPS, especially after the thermophilic phase. The hydrophilic amino acid content decreased by 23.02%, verifying that photocatalysis could improve EPS hydrophobicity. Consequently, municipal sludge biodrying coupled with photocatalysis promotes sludge EPS degradation and enhances sludge dewaterability, improving the efficiency of sludge biodrying.

Mechanism insights into hydrothermal dewatering of food waste digestate for products valorization

Poor dewaterability is a bottleneck of the disposal of digestate from food waste (DFW). However, the dewatering mechanism remains unclear due to the complex composition of DFW. Understanding the dewatering mechanism, as well as the transformation of organic/inorganic matters is essential for the DFW management and valorization. In this study, the distribution, transformation, and complex interplay of organic and inorganic matters at different Hydrothermal treatment (HTT) temperatures were comprehensively analyzed to explore the hydrothermal dewatering mechanism of DFW. When HTT was conducted in the temperature range of 120-180 °C, the interstitial water was released as surface or free water because of membrane breaking and size reduction of the solid substrate. Releasing divalent cations increased the Zeta potential of the bulk solution. The weaker electrostatic repulsion between suspended particles made them easier to settle as the centrifugation cake. When the temperature of HTT was above 180 °C, polymerization and aromatization reactions took place gradually for organic matters, and the bound water was further removed. The generated humic substances were more hydrophobic than the raw material. In addition, the humic substance could combine with cationic metals, which decreased the zeta potential of the bulk solution but promoted the aggregation of nanoparticles and enhance the dewaterability of DFW.

Enhancement of sewage sludge dewaterability by fungal conditioning with Penicillium simplicissimum NJ12: from bench- to pilot-scale consecutive multi-batch investigations

Bench- and pilot-scale successive multi-batch trials were conducted to investigate the performance and sustainability of fungal conditioning with Penicillium simplicissimum NJ12 for improving sludge dewatering. The dominant factors affecting the sludge dewaterability improvement by P. simplicissimum NJ12 were also identified. Fungal treatment with P. simplicissimum NJ12 at a volume fraction of 5% of the inoculum greatly improved the sludge dewaterability. This improvement was characterized by sharp decreases in the specific resistance to filtration from 1.97 × 10¹³ to 3.52 × 10¹¹ m/kg and capillary suction time from 32 to 12 s within 3 days. Stepwise multiple linear regression analysis showed that a marked decrease (58.8%) in the protein content in slime extracellular polymeric substances and an increase in the zeta potential of the sludge (from -35 to -10 mV) were the most important factors that improved the dewaterability of sludge after fungal treatment. Consecutive processes of fungal treatment could be realized by recirculating the fungal-treated sludge with a recycling rate of 1:2 (V_{biotreated sludge}/V_{total sludge}). The treatment effectiveness was maintained only over three successive cycles, but replenishment with fresh P. simplicissimum NJ12 would be provided periodically at set batch intervals. These findings demonstrate the possibility of P. simplicissimum NJ12-assisted fungal treatment for enhancing sludge dewatering.

Environmental applications of microbial extracellular polymeric substance (EPS): A review

During the last decade, water demand and wastewater generation has increased due to urbanization around the globe which had led to an increase in the utilization of chemicals/synthetic polymers for treating the wastewaters. These synthetic polymers used during the coagulation/flocculation process are non-renewable, non-biodegradable, and have a potential neurotoxic and carcinogenic effect. From the literature it is clear that extracellular polymer substance (EPS) is a potential bioflocculant, moreover it is renewable, biodegradable, eco-friendly, non-toxic as well as economically valued product. The various identification techniques and extraction methods of EPS are elaborated. Further application of EPS as absorbent in removing the dye from the industrial effluent is presented. Moreover EPS as a potential adsorbent for heavy metal removal from the various effluent is discussed. In addition, EPS is also utilized for soil remediation and soil erosion control. Mainly, EPS as bioflocculant in treating raw water, wastewater treatment, leachate and sludge management are summarized in this review.

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

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

Insight into formation and biological characteristics of Aspergillus tubingensis-based aerobic granular sludge (AT-AGS) in wastewater treatment

The long start-up time and facile biomass loss of aerobic granular sludge (AGS) impede its application for actual wastewater treatment. The present study investigates a novel assist-aggregation strategy based on Aspergillus tubingensis (AT) mycelium pellets to accelerate sludge granulation, and engineered Fe₃O₄ nanoparticles (NPs) were used to further enhance flocculent sludge (FS) aggregation. The AT mycelium pellets, modified by 0.5 g/L Fe₃O₄@SiO₂-QC NPs (AT-V), had a more compact internal structure than the unmodified group (AT-I). The content of extracellular polymeric substances (EPS) and the zeta potential values were observed to increase from 39.86 mg/gVSS and -9.19 mv for AT-I to 69.64 mg/gVSS and 2.35 mv for AT-V, respectively. In optimized cultivation conditions, the aggregated sludge biomass of AT-V reached 1.54 g/g. An original AT-based AGS (AT-AGS) with a high biological activity (64.45 mgO₂/gVSS·h as specific oxygen uptake rate) and enhanced velocity (58.22 m/h) was developed in only 9 days. The removal efficiencies of total nitrogen (TN) and total phosphorus (TP) of the AT-AGS were 12.24% and 16.29% higher than those of the inoculated FS under high feeding load. Additionally, the analysis of cyclic diguanylate (c-di-GMP) and con-focal microscope images implied that polysaccharide (PS) of EPS played an important role in maintaining the stability of the AT-AGS. Finally, the dominant functional species contributing to sludge aggregation and pollutants removal of the AT-AGS showed a larger richness and diversity than those of the inoculated FS. This study might provide a novel high-efficiency strategy for the fast formation of AGS.

Protein extraction from excess sludge by alkali-thermal hydrolysis

The protein in excess sludge can be extracted effectively by the alkali-thermal method, and the extracted protein can be used as a foaming agent and in other industrial raw materials to realize its resource utilization. In this paper, the factors influencing sludge protein extraction by the alkali-thermal method were optimized based on the protein extraction rate and the polypeptide content, which determine the foaming performance of the extracted protein. The results showed that the optimal conditions were a pH of 12, a temperature of 120 °C, a reaction time of 4 h, and a sludge moisture content of 92%. Under these optimized conditions, the extraction rate of protein and the concentration of polypeptides were 88.3% and 6599 mg/L, respectively. Additionally, the foaming performance of the extracted protein solution was tested, and the foamability and foam stability were close to 450% and 88.8%, respectively. Therefore, the sludge protein extracted by the alkali-thermal method can meet the relevant standards of foam extinguishing agents and concrete foaming agents in China. In addition, the dewatering performance of the hydrolyzed sludge was improved by 93.1%, which provided favorable conditions for the subsequent separation of the protein solution.