Research progress in improving sludge dewaterability: sludge characteristics, chemical conditioning and influencing factors

Evaluating the contact angle as an indicator of sludge dewatering performance: Limitations and methodological insights

This study evaluates the utility of contact angle (CA) as an indicator of sludge dewatering performance, addressing inconsistencies in previous findings and the lack of a clear standard. We systematically compared four pretreatment methods-agar coating, in-situ filtration, lyophilization, and hot air drying-to assess contact angle efficacy. Measurement reliability was determined using standard deviation and coefficient of variation (CV), revealing that hot air drying (74.85° ± 4.24°, CV 5.67 %) and lyophilization (71.48° ± 4.74°, CV 6.6 %) exhibited significantly lower variability than agar coating (42.54° ± 12.24°, CV 28.7 %) and in-situ filtration (53.35° ± 9.66°, CV 18.1 %). Correlation matrix and Variable Importance in Projection (VIP) analyses indicated that contact angle only partially correlates with dewatering performance indicators, including water content, capillary suction time (CST), and specific resistance to filtration (SRF). Importantly, the predictive power of CA measurements varies across different pretreatment methods, underscoring its limitations as a standalone metric. Over-reliance on CA measurements may lead to biased conclusions. This study offers key perspectives on refining CA measurement methods and highlights the importance of comprehensive evaluation of sludge dewatering performance, guiding the development of more efficient treatment technologies.

Enhanced deep-dewatering of high EPS-Containing sludge waste using PDMDAAC-PFS and H2O2

Large quantities of sludge from wastewater treatment plants require dewatering before final disposal, with extracellular polymeric substance (EPS) significantly impacting sludge dewatering performance. This study achieved deep-dewatering of sludge with high EPS content (164.36-181.62 mg/g VSS) by reducing moisture content to 57.2% using a combination of polydimethyldiallylammonium chloride-ferric sulphate (PDMDAAC-PFS) and hydrogen peroxide (H2O2). Using PDMDAAC-PFS alone decreased tightly bound EPS (TB-EPS) from 152.7 to 135.3 mg/g VSS while increasing EPS by 77.2%, indicating that the reagent solubilised TB-EPS and disrupted the cell wall, releasing intracellular bound water. Further addition of H2O2 reduced total EPS by 34.2-55.1%, primarily affecting extracellular protein (PN) in TB-EPS. Three-dimensional fluorescence and Fourier transform infrared spectrometry revealed that the reduced PN in TB-EPS consisted mainly of tyrosine proteins, with the destruction of hydrophilic groups (amino group and carbonyl group) contributing to the release of bound water. Quenching tests demonstrated that hydroxyl radicals generated by H2O2 were crucial for EPS oxidation, contributing to 21.9% of sludge dewatering. Additionally, PDMDAAC-PFS re-flocculated the structurally damaged sludge into larger, more porous particles, enhancing the dewatering process. Thus, this study presents a novel method of deep-dewatering for waste sludge containing high EPS content.

Treatment of water discharged from a Yellow River water purification plant: optimization and application of enhanced coagulation technology

In this study, the effects of the coagulant type, compounding ratio, dosage, hydraulic conditions and water temperature on the intensive coagulation process were systematically investigated for the treatment of discharge water with high turbidity and high sediment characteristics from a Yellow River water purification plant (YRWPP). Results indicated that the PFS-PDMDAAC composite coagulant achieved optimal treatment performance under the following conditions: a blending ratio of 6 : 1, a dosage of 13.65 mg L-1, rapid mixing at 300 rpm for 1.5 min, slow mixing at 120 rpm for 7 min, and a water temperature of 20 °C. Employing these parameters, supernatant turbidity was reduced to 46.2 NTU, the specific resistance to filtration (SRF) of the sludge was less than 0.94 × 1012 m kg-1, and the solid content exceeded 8%, enabling direct dewatering. With the increase of the concentration of the discharged mud water, the effect of enhanced coagulation is weakened. Specifically, when the concentration was lower than 4.14%, a dosage of 13.65 mg L-1 could be treated effectively; when the concentration was in the range of 4.14-9.12%, the dosage needed to be increased to 21 mg L-1; and when the concentration was more than 9.12%, it was difficult to achieve the discharge standard. Furthermore, zeta potential analysis showed that the absolute zeta potential value was the lowest when the compounding ratio was 6 : 1, and the effect of colloid destabilization was the best. According to floc morphology observation, the volume of floc increased and the structure of floc was loose and porous after enhanced coagulation, which was conducive to settling and dewatering. The analysis of the floc particle size showed that the average floc particle size increased from 38.3 μm to 238 μm at a dosage of 21 mg L-1, and the settling performance was significantly improved; when the dosage was more than 21 mg L-1, the floc particle size decreased, and the settling performance declined. This study provides technical support for the treatment and resource utilization of mud water discharged from the Yellow River water purification plant.

Unveiling the mechanism and driving factors of pharmaceutical and personal care product (PPCP) removal in wastewater treatment plants

Wastewater treatment plants (WWTPs) are primary point sources of pharmaceuticals and personal care products (PPCPs) entering the environment; however, few studies have systematically elucidated the PPCP removal mechanism in WWTPs. In this study, we conducted two composite sampling campaigns, collecting water and sludge samples from each treatment stage of four secondary or tertiary WWTPs with various processes. Our goal was to identify the mechanisms and driving factors behind the removal of 30 common PPCPs. The average removal efficiency of all PPCPs was 62.57 %, with significant variations (-308.03 %-91.03 %) among individual PPCPs. The contribution of sludge adsorption, biodegradation and chemical degradation to the removal of 30 PPCPs was quantified. The average biodegradation efficiency of sulfonamides was 44.90 %, but reconversion of chelate products to the sulfonamides after chemical treatment (UV) was the main reason for their low removal efficiency (about 30 %). Base dissociation constant (pKb) and logKow were used to evaluate the contribution of charge interactions and hydrophobic partitioning to the adsorption capacity of PPCPs for the first time. For PPCPs that could ionize into cations, higher pKb increased adsorption capacity, whereas for other PPCPs logKd (distribution coefficient) and logKow showed a significant positive correlation. The biodegradation of sulfonamides was positively correlated with their solubility. The presence of hydroxyl and carboxyl groups promoted microbial degradation of non-antibiotic compounds. This study reveals the universal mechanisms and driving factors behind PPCP removal in WWTPs, providing insights to guide the targeted optimization of treatment processes for PPCP removal.

The impact of radicals on physicochemical properties of waste activated sludge during hydrodynamic cavitation treatment

In this study, laboratory-scale Pinned Disc Rotary Generator of Hydrodynamic Cavitation was used to treat waste-activated sludge with a Total Solids concentration of 0.7 %. Five different rotor-stator arrangements were tested, focusing on waste-activated sludge physicochemical and rheological parameters of industrial relevance: general chemical analysis, rheometry, dewaterability, interfacial tension, UV-Vis and FTIR spectroscopy. Radical formation in all five arrangements was confirmed using salicylic acid dosimetry before sample testing. Three of the arrangements generated twice the radical concentration of the other two and achieved a disintegration degree three times higher (17 % compared to 5 %). Capillary Suction Time tests demonstrated a 14-fold reduction in filterability across all arrangements, accompanied by an increase in interfacial tension exceeding 10 %. Statistically significant changes in the UV-Vis spectra indicated alterations in dissolved organic matter humification, aromaticity, and molecular size of colorimetric dissolved organic matter, DNA, and RNA. FTIR analysis revealed characteristic peaks at 1537 cm-1 and 1648 cm-1, signifying microbial cell wall damage. Rheological analysis showed a reduction in apparent viscosity within the low shear stress zone (τ < 5 Pa) and a shift in the yield stress point to lower shear stresses (τ < 0.14 Pa compared to τ = 0.17 Pa for the untreated samples). Pearson's correlation test revealed strong, statistically significant correlations between cell wall damage (as identified by FTIR) and hydrodynamic conditions in the reactor, while the correlation with radical formation was not statistically significant. This suggests that hydrodynamic forces were the primary drivers of cell wall damage, with potential synergetic effects from radicals.

Reuse of wastewater and biosolids in soil conditioning: Potentialities, contamination, technologies for wastewater pre-treatment and opportunities for land restoration

This study reviews the potential use of various wastewaters-vinasse, swine, food industry, paper and pulp, municipal wastewaters, and biosolids-as soil conditioners for restoring degraded areas, focusing on the circular economy concept. Over 90 articles from 2013 to 2024 were analyzed to address current scientific concerns, including these effluents' resistance genes, hormones, and macro/micronutrients. The presence of contaminants was critically examined alongside the necessary treatment methods to prevent soil degradation and ensure soil quality improvement. These included contaminants of emerging concern (CECs), antibiotic resistance genes (AGRs), and pathogens. These contaminants can either be assimilated and degraded by the soil ecosystem or leach into groundwater, translocate to plants, or accumulate in surface soil, necessitating careful monitoring. Furthermore, the study critically evaluates the potential of various physical and biological treatment technologies, such as anaerobic digestion, composting, dewatering, stabilization ponds, biological reactors, membrane processes, rotating disks, and pelletizers, highlighting their effectiveness in mitigating contamination and enhancing soil quality. The long-term effects of wastewater reuse as soil conditioner depend on both wastewater characteristics and soil properties. The benefits of using wastewater as soil conditioners are found to be influenced by characteristics of both the soil and the wastewater, with improvements in soil physical properties (increased porosity and permeability) and chemical properties (increased soil organic carbon and nutrients). Overall, the literature suggests that while wastewaters hold promise as soil conditioners, their successful application depends on effective wastewater management strategies to optimize benefits and mitigate risks.

Sewage sludge valorization via phytohormones production: Parameter regulation and process evaluation

Sludge treatment is of great significance for environmental protection and sustainable development. Existing treatment technologies fall short in terms of carbon emissions, process efficiency, and resource recovery. This study focuses on alkaline hydrothermal treatment, proposing a short-cycle, low-energy, high-value management process for sludge valorization. Here, we investigate the impact of treatment duration, temperature, and solid content on the synthesis of high-value products and their effects on both solid and liquid phases. Based on the comprehensive results, 2 h, 160 °C, and 14 % solid content can be regarded as the optimized treatment condition. The resulting products, including phytohormones, humic substances, and essential nutrients (C, N, P and K), exhibit substantial potential for high-value agricultural utilization. In the unconcentrated solution, a single phytohormone can reach a concentration of 104 μg/L. Heavy metal content is well below standard limits, simultaneously achieving biological stability, and the volume can be reduced to 60 %. This process is 42.12 times more energy-efficient than conventional anaerobic digestion. This novel approach promotes waste resource recycling and sustainable urban management.

Unlocking the potential of environmentally friendly adsorbent derived from industrial wastes: A review

With increasing urbanization and industrialization, growing amounts of industrial waste, such as red mud (RM), fly ash (FA), blast furnace slag (BFS), steel slag (SS), and sludge, are being produced, exposing substantial threats to the environment and human health. Given that numerous researchers associate with conventional adsorbents, developing and utilizing industrial wastes derived from adsorption technology still has received limited attention. Utilizing this waste contributes to developing alternative materials with superior performance and significantly reduces the volume of solid waste. The excellent physical and chemical characteristics of these wastes are also investigated in this paper. This review attempts to demonstrate a comprehensive overview of the application of industrial waste-based adsorbent in the adsorption process for removing organic pollutants, dyes, metallic ions, non-metallic ions, and radioactive substances. In addition, industrial waste-based adsorbents are among the most promising and applicable techniques for pollutant removal, offering remarkable adsorption efficiency, rich surface chemistries, reasonable cost, simple operation, and low energy consumption. This review summarizes state-of-the-art advancements in engineered adsorbents (including physical and chemical modifications). It provides a holistic view regarding a comprehensive understanding of the mechanism involved in adsorption for water remediation. The challenges and the prospects for future research in applying these adsorbents are also elucidated, contributing to sustainable waste management and environmental sustainability.

Plant-Based Flocculants as Sustainable Conditioners for Enhanced Sewage Sludge Dewatering

With the aim to establish clean and sustainable sludge treatment, green conditioning using natural flocculants has recently gained a growing interest. In this study, a variety of plant materials, namely Moringa (Moringa oleifera) seeds, Fenugreek (Trigonella foenum-graecum) seeds, Potato (Solanum tuberosum) peels, Aloe (Aloe vera) leaves, Cactus (Opuntia ficus indica) cladodes, and Phragmites (Phragmites australis) stems, were evaluated for their potential bioflocculant activity in conditioning sewage sludge. They were thoroughly characterized to determine their active flocculating compounds. Sludge dewaterability was evaluated by assessing various sludge parameters, including specific resistance to filtration (SRF), dryness of filtration cake (DC), and total suspended solid removal (TSS) from sludge filtrate. The collected results from various physicochemical characterizations of plant materials suggest that the main flocculating agents are carbohydrates in Cactus and Fenugreek and proteins in Moringa, Potato, and Phragmites. Additionally, all tested plant-based flocculants demonstrated effective dewatering performance. Interestingly, compared to the chemical flocculant polyaluminum chloride, Moringa and Cactus showed superior conditioning effects, yielding the lowest SRF values and the highest DC. As a result, the use of these natural flocculants improved sewage sludge filterability, leading to a significant removal of total suspended solids from the filtrate. The conditioning properties of Moringa and Cactus can be attributed to their high protein and sugar content, which facilitates the effective separation of bound water from solids through charge neutralization and bridging mechanisms. Thus, green conditioning using plant-based flocculants, particularly Moringa and Cactus materials, presents a promising and eco-friendly approach to enhance sewage sludge dewatering for safer disposal and valorization.

Enhancement of alkaline pretreatment-anaerobically digested sludge dewaterability by chitosan and rice husk powder for land use of biogas slurry

Alkaline pretreatment can improve the methane yields and dewatering performance of anaerobically digested sludge, but it still needs to be coupled with other conditioning methods in the practical dewatering process. This study utilized four different flocculants and a skeleton builder for conditioning of alkaline pretreatment-anaerobically digested sludge. Chitosan was found to be the most effective in dewatering the sludge. Chitosan coupled with rice husk powder further improved the dewatering performance, which reduced normalized capillary suction time, specific resistance to filtration, and moisture content by 98.7%, 82.0%, and 12.1%. For land use of biogas slurry as a fertilizer, chitosan conditioning promoted the growth of corn seedlings, while the other three flocculants diminished the growth of corn seedlings. Chitosan coupled with rice husk powder further promoted the growth of corn seedlings by 103.5%, 65.0%, and 53.7% in fresh weight, dry weight, and root length, respectively. Overall, chitosan coupled with rice husk powder not only enhanced the dewaterability of alkaline pretreatment-anaerobically digested sludge but also realized the resource utilization of agricultural waste.

Molecular simulation of the slurrying mechanism in microplastic semi-coke water slurry

CONTEXT

This study explores the interaction between particles in microplastic semi-coke water slurry at the molecular level using molecular simulation methods, specifically DFT calculations and MD simulations. In addition, the experiment of slurry preparation was carried out to study the viscosity and stability of the slurry. The electrostatic potential analysis shows that the interaction between microplastics and dispersant molecules occurs on atoms with large electronegativity or oxygen-containing functional groups, and the energy gap of frontier molecular orbitals indicated that PVC interacts most easily with the dispersant (0.39 eV), followed by PS (1.08 eV) and PET (3.65 eV). In addition, it is also noted that due to the steric hindrance effect, the adsorption energy was opposite to the DFT calculation results: PET was - 213.338 kcal/mol (NNO) which was highest, followed by PS (- 107.603 kcal/mol, NNO), and PVC (NNO) was lowest which was - 94.808 kcal/mol. And RDF shows similar results, which the probability of water molecules in the PET system was the highest, followed by PS, and finally, PVC. The MD results are consistent with the viscosity and stability characterization results of the slurry which PET has the lowest viscosity of 87.3 mPa·s. Finally, this study provides new ideas for the treatment of microplastics and the improvement of the performance of semi-coke water slurry and reveals the interaction mechanism between microplastics and semi-coke water slurry.

METHODS

All calculations were performed using Materials Studio (MS) version 2020 software, BIOVIA Corporation. The DFT calculation was carried out through the DMol3 module. The DFT calculations include electron density, electrostatics, orbitals, and population analysis. In DMol3 module, the GGA-PBE function was selected to consider gradient changes in density in the simulated calculation. The DFT-D correction was selected, and all electrons were calculated by DNP for accurate core potentials and the DNP file was 4.4. MD simulation was performed through the Forcite module. MD simulation mainly focuses on relative concentration distribution analysis, radial distribution function, and adsorption energy calculation. All molecular geometry optimizations are performed in the Forcite module. In the molecular dynamic part, all simulations used PCFF forcefield. The NVT ensemble was adopted and using the Nosé thermostat.

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.