Key Takeaways & Executive Findings
- •• • Cement addition significantly boosts early and mid-term strength of solidified sludge; loess dominates later strength gain, with optimal mix ratio loess:fly ash:desulfurized gypsum:cement:sludge = 0.1:0.12:0.12:0.08:1 (by mass). • • Optimal fly ash and desulfurized gypsum content is 12% each; higher contents cause particle agglomeration, reducing compressive strength. • • XRD and SEM confirm formation of C-S-H gel and ettringite (Aft), which densify the matrix and effectively immobilize heavy metals, reducing leachability. • • The optimal solidifying agent composition (by mass) is 23.81% loess, 28.57% fly ash, 28.57% desulfurized gypsum, and 19.05% cement, with total solidifying agent dosage around 40% of sludge mass, balancing cost and performance.
Abstract
To address land waste and poor bearing capacity from sludge landfill, this study developed a composite solidifying agent using loess, fly ash, desulfurized gypsum, and cement. Orthogonal experiments combined strength testing, SEM/XRD microanalysis, permeability and heavy metal leaching tests, and cost accounting. Results show that cement significantly enhances early and mid-term strength, while loess dominates later strength development. Optimal fly ash and desulfurized gypsum content is 12% each. The optimal mix ratio (loess:fly ash:desulfurized gypsum:cement:sludge) is 0.1:0.12:0.12:0.08:1. The solidified matrix forms dense structures via C-S-H gel and ettringite (Aft), effectively controlling heavy metal leaching at low cost. This work enables solid waste resource utilization and provides robust support for sludge solidification engineering.
1. Introduction
Municipal sludge generation has risen sharply with urbanization, and landfill remains the most mature disposal method. However, increasing sludge volumes cause land waste, poor bearing capacity, and ecological degradation at landfill sites. Conventional cement solidification alone yields porous, weak products even at high dosages, necessitating composite solidifiers that combine cement with supplementary materials like lime, fly ash, and bentonite to enhance mechanical properties and contaminant immobilization.
This study leverages abundant loess resources in northwest China, combined with industrial by-products (fly ash and desulfurized gypsum) and Portland cement, to formulate a green, low-carbon solidifying agent. Through orthogonal experiments, the optimal mix ratio is identified, and the underlying mechanisms are elucidated via XRD and SEM. This approach not only reduces solidification costs but also promotes industrial waste recycling, addressing the dual challenges of sludge disposal and resource efficiency.
Loading authentic research manuscript (Pages 1–5)...
MA Xiaolei, BAI Xuemei, LI Jian, SHI Xujun, YE Shuaihua, JU, SHEN Zhiyuan, SHI Hongzhuang (2026). Strength and Microstructural Characteristics of Sludge Solidified by Loess-Based Composite Solidifying Agent. Chinese Journal of Environmental Engineering. https://doi.org/10.12030/j.cjee.202507094
Research & Educational Purpose Only: The translations, structured abstracts, analytical annotations, and data reports provided by SinoGreenTechare intended exclusively for academic research, internal corporate R&D, and educational benchmarking. They do not constitute formal engineering, chemical safety, legal, or professional advice.
Copyright & Intellectual Property Notice: Original copyright of the underlying source articles and experimental data remains with the respective authors, institutions, and original publishing journals. SinoGreenTech claims intellectual property only over its proprietary translations, analytical syntheses, and AEO structured enhancements in accordance with international fair use and academic citation principles.
Frequently Asked Questions
What is the optimal mix ratio for the loess-based composite solidifying agent, and what are the corresponding strength and cost benefits?
The optimal mix ratio (loess:fly ash:desulfurized gypsum:cement:sludge) is 0.1:0.12:0.12:0.08:1 by mass, with a total solidifying agent dosage of approximately 40% of sludge mass. This formulation achieves significant strength improvement and effective heavy metal immobilization at lower cost compared to cement-only solidification, as demonstrated by the study's cost analysis.
How does the addition of fly ash and desulfurized gypsum affect the solidification performance, and what is the optimal dosage?
Fly ash and desulfurized gypsum act as active mineral admixtures that promote cement hydration and form additional C-S-H and ettringite. The optimal dosage is 12% each; beyond this, particle agglomeration occurs, reducing compressive strength. This is evidenced by the orthogonal test results showing strength peaks at 12% content.
What are the key microstructural mechanisms responsible for strength development and heavy metal immobilization in the solidified sludge?
XRD and SEM analyses reveal that cement hydration produces C-S-H gel and ettringite (Aft), which fill pores and bind sludge particles, creating a dense matrix. Loess absorbs water and acts as a filler, further densifying the structure. These phases also encapsulate heavy metals, reducing their leachability.
How does the initial moisture content of sludge influence the solidification outcome?
Higher initial moisture content leads to a more porous and less dense solidified structure, which reduces compressive strength. The study indicates that the solidified sludge becomes increasingly loose as moisture content rises, negatively impacting strength development.
What are the environmental and economic advantages of using this composite solidifying agent compared to traditional cement-only methods?
The composite agent utilizes industrial by-products (fly ash, desulfurized gypsum) and locally available loess, reducing cement consumption and overall material costs. It also effectively immobilizes heavy metals, minimizing environmental risks. The study reports lower cost while achieving comparable or superior strength, making it a sustainable alternative for sludge solidification.
Related Chinese Research & Cross-Citations
Analysis of National and Local Policies for Medical Waste Treatment and Disposal in China
The escalating generation of medical waste, driven by healthcare expansion and frequent medical activities, poses significant environmental and public health risks. Under the framework of ecological civilization, China is developing a comprehensive policy system for medical waste treatment and disposal, yet the current framework remains nascent and exhibits inconsistencies between national and local policies. This study systematically analyzes the status of national and local policies from 2003 to 2024, collecting 413 policy documents (166 from national ministries and 247 from provincial governments). The analysis examines temporal evolution, regional distribution, and policy focus, alongside the influence of medical waste output, treatment technologies, facility infrastructure, and major epidemic responses. Findings reveal distinct policy phases: initial self-disposal, exploratory management, foundational system building, and rapid development. Regional disparities are pronounced, with eastern coastal areas showing more advanced policies due to greater technical and financial resources. The surge in medical waste, particularly during the COVID-19 pandemic, underscores the need for enhanced regulatory guidance. Non-incineration technologies are gaining traction for their environmental and cost benefits, and facility coverage has improved but remains uneven. The study proposes five policy principles to foster technological innovation and industrial upgrading, ensuring safe medical waste management and environmental protection.
Kinetic Analysis and Simulation of Pollutant Removal in Sewage Networks
This study investigates pollutant removal characteristics and kinetic behaviors in sewage networks, and analyzes their impact on the carbon-to-nitrogen ratio (C/N, as COD/TN) of influent to wastewater treatment plants. Source water quality sampling at drainage outlets revealed spatial and temporal variations in C/N, with domestic sewage exhibiting higher C/N than industrial sewage, and diurnal peaks reaching 6.92 versus 4.71 during off-peak hours. Using a pilot-scale adjustable sewage network system in Kunshan, experiments were conducted under high (0.491 m·s−1) and low (0.089 m·s−1) flow velocities, monitoring pollutant removal over 144 hours. Pseudo-first-order kinetics were applied to model COD and TN removal. Results showed that COD (including SCOD and PCOD), BOD5, and SS achieved approximately 80% removal within 144 h, with higher removal at low flow velocity. TN, NH3-N, and TP exhibited lower overall removal rates. Kinetic fitting revealed that COD removal rate constants (kCOD) were significantly higher than those for TN (kTN), and both decreased with increasing flow velocity: at low velocity, kCOD=0.0167 h−1 and kTN=0.0029 h−1; at high velocity, kCOD=0.0127 h−1 and kTN=0.0020 h−1. Simulations based on actual source pollutant concentrations indicated that the time for C/N to drop to the denitrification critical value of 4.50 was 12.24 h at high velocity, but shortened to 9.49 h at low velocity. These findings demonstrate that increasing flow velocity effectively retards the decline of C/N. Therefore, regulating network flow velocity to reduce hydraulic retention time is a key strategy for maintaining adequate C/N at the terminal and ensuring denitrification efficiency in wastewater treatment plants.
Preparation of Trimetallic-Carbon Composite Catalysts and Their Application in Catalytic Ozonation of Industrial Wastewater
Advanced oxidation processes (AOPs) are promising for degrading organic pollutants in water treatment. Heterogeneous catalytic ozonation (HCO) has gained attention due to its high oxidation efficiency, strong interference resistance, and low secondary pollution. In this study, a series of trimetallic-carbon composite ozone catalysts were prepared via an organic precursor calcination method using γ-Al2O3 as support. This method enhanced catalytic activity and mechanical strength while overcoming the limitations of carbon materials (low mechanical strength) and metal-based materials (poor mass transfer). The optimized catalyst, CA-FeCoCu, comprising Fe, Co, Cu, carbon, and alumina, exhibited excellent performance in phenol degradation and real industrial wastewater treatment. Characterization revealed that the synergistic effect of trimetals and the introduction of multiple carbon types increased specific surface area and hydroxyl radical (·OH) generation. In a pilot-scale fixed-bed reactor, the CA-FeCoCu/O3 system reduced COD from 120 mg·L−1 to below 40 mg·L−1, with an O3 consumption ratio (O/C) of less than 1, effectively lowering operational costs. This work provides a new strategy for developing efficient and stable heterogeneous O3 catalysts and offers a reference for the practical application of HCO in industrial wastewater treatment.
Differentiated Characteristics of Suspended Particulate Matter and Their Effects on Water Quality in the Middle and East Routes of the South-to-North Water Diversion Project
This study investigates the spatiotemporal differentiation of suspended particulate matter (SPM) characteristics, sources, and their impacts on water quality between the Middle Route (closed artificial channel) and East Route (open natural water system) of the South-to-North Water Diversion Project. Thirty sampling sites (13 on the Middle Route, 17 on the East Route) were established, and samples were collected during dry and wet seasons. Water quality parameters and SPM characteristics were analyzed, including particle size distribution, total suspended solids (TSS), chlorophyll a, and stable carbon and nitrogen isotopes. Results show that the Middle Route maintains good and stable water quality, with SPM dominated by coarse particles (>63 μm, 61.43%–94.68%), total phosphorus (TP) <0.01 mg·L−1, and a significant positive correlation between chlorophyll a and coarse particles (r=0.60), indicating algal aggregation dominates particle formation. In contrast, the East Route exhibits high and fluctuating nitrogen and phosphorus concentrations, with SPM dominated by fine particles (<20 μm, 51.26%–88.61%), TP ranging from 0.03 to 1.11 mg·L−1, and a positive correlation with fine particles, suggesting significant external inputs. Carbon and nitrogen isotope analysis reveals that Middle Route SPM primarily originates from autochthonous algae (contribution >46.75%), while East Route SPM is influenced by both terrestrial C3 plants and algae. The distinct engineering and management approaches of the two routes lead to significant differences in SPM characteristics and sources, thereby affecting water quality dynamics. The Middle Route requires an 'algal reduction and hydrodynamic optimization' strategy to control algal-derived coarse particle deposition, whereas the East Route benefits from 'retention-sedimentation and wetland purification' to reduce external fine particles and pollutant inputs. This research provides theoretical support and practical guidance for differentiated SPM management in long-distance water diversion systems.
Combined Ozone Micro-Nano Bubble Oxidation and Powdered Activated Carbon Adsorption for Removal of Taste and Odor Compounds from Drinking Water
Algal-derived taste and odor compounds (2-methylisoborneol, 2-MIB, and geosmin, GSM) in drinking water sources are poorly removed by conventional treatment. This study systematically evaluated the standalone and combined performance of ozone micro-nano bubbles (O3-MNBs) oxidation and powdered activated carbon (PAC) adsorption for removing 2-MIB, GSM, and algal cells from source water. Results showed that O3-MNBs pre-oxidation achieved >97.5% removal of odorants at 400 ng·L−1 and 67.2% algal cell removal within 30 min. When applied as a deep treatment stage, the degradation rate constant (k) was 10.1%–25.6% higher than in pre-oxidation due to lower background matrix interference. Both pre-oxidation and deep treatment reduced effluent concentrations of 2-MIB and GSM to below 10 ng·L−1, with oxidation kinetics fitting pseudo-first-order models (R²>0.95). PAC adsorption of both compounds followed pseudo-second-order kinetics (R²>0.99), with GSM equilibrium adsorption capacity approximately 20.0% higher than that of 2-MIB. In pure water, adsorption capacity increased by >10.0% compared to raw water. Based on kinetic models, a quantitative prediction method was established for O3-MNBs oxidation and PAC adsorption processes, aiming to achieve efficient odorant removal and cost optimization, providing theoretical support for advanced drinking water purification and smart water plant construction.
Pulsed Electric Field Enhancement of Nitrogen Removal Performance and Microbial Community Structure Response in Anammox Granular Sludge
This study investigated the effects of a ring-shaped pulsed electric field (PEF) (1.5 V, 4 h on-time per cycle) on nitrogen removal performance and microbial community structure of anammox granular sludge (AnGS). Two anaerobic sequencing batch reactors (R1 control, R2 with PEF) were operated under stepwise increasing nitrogen loading rates (NLR). At NLR below 1,155 mg·(L·d)−1, R2 exhibited total nitrogen removal efficiency (TRE) 7.5%–17.0% higher than R1, with biomass, specific anammox activity (SAA), and extracellular polymeric substances (EPS) increased by 5%–7%, 21%–71%, and 54%–77%, respectively. However, at NLR above 1,320 mg·(L·d)−1, the toxic effect of nitrite dominated, and PEF enhancement diminished or even reversed to inhibition. Microbial community analysis revealed that at low-to-moderate NLR, PEF increased the relative abundance of Planctomycetes and key anammox bacteria (Candidatus Brocadia and Candidatus Jettenia), along with enhanced community richness (Chao1) and diversity (Shannon/Simpson indices). At high NLR, PEF decreased microbial richness compared to R1. Principal component analysis and redundancy analysis indicated that PEF was the key factor driving community differences at low-to-moderate NLR, whereas nitrite concentration became the dominant factor at high NLR. This study provides theoretical support for enhancing the resilience and engineering application of anammox processes.