FENG Shuangxi^1a，1b，1c，ZHANG Guoqing^1a ，LEI Huayang1a，^1b，2，3，LI Jiankai^1a ，YU Shengpeng³

1a. School of Civil Engineering，1b. Key Laboratory of Coast Civil Structure Safety of the Ministry of Education，1c. Key Laboratory of Earthquake Engineering Simulation and Seismic Resilience， China Earthquake Administration， Tianjin University， Tianjin 300350， China；

2. School of Civil Engineering and Architecture， Guangxi University， Nanning 530004， China；

3. China Railway 20th Bureau Group Co. ， Ltd. ， Xi'an 710016， China

Abstract

Significance This paper explores the application and development of chemical agent-assisted vacuum preloading in sludge dewatering treatment. It aims to optimize chemical agent selection and dosage to enhance sludge dewatering efficiency， improve soil consolidation， and reduce treatment costs.

Progress Sludge composition and microstructure are comprehensively reviewed in the study. Sludge is a complex mixture of organic pollutants， viruses， inorganic minerals， microplastics， and heavy metals， and it is characterized by a stable flocculent structure. Based on the mechanical properties of sludge from landfills in Shanghai and Shenzhen， the variation patterns of sludge parameters were analyzed， including organic matter content， particle size distribution， water content， permeability coefficient，compressibility， and strength. The organic matter content ranges from 20. 1% to 52. 2%， with an average value of 45%. The water content varies from 190% to 630%， with an average of 500%. The permeability coefficient is within the range of 0. 52×10⁻⁸ to 1. 7×10⁻⁸ cm/s. These data demonstrate that sludge is characterized by high water content and poor permeability. The water in sludge exists in the states of free water， capillary water， and bound water. Common dewatering methods include centrifugal dewatering， filter-press dewatering， electrokinetic dewatering， and thermal dewatering， with the dewatering efficiency increasing from the former to the latter. Vacuum preloading， adapted from silt foundation reinforcement， was initially studied in 1957 by Chinese researchers and has been widely adopted since 1995. During the drainage process when using this method， soil particles migrate directionally and uniformly towards the drainage board. Clay particles accumulate around the drainage board，forming a clogging layer that impairs drainage. To address these limitations， chemical agents are introduced to sludge treatment.To promote particle aggregation and enlarge sludge particle size， inorganic and organic flocculants are commonly employed. Oxidants such as Fenton’s reagent are utilized to break down extracellular polymeric substances （EPS） and release bound water，thereby enhancing drainage volume. Alkaline materials are used to facilitate chemical modification and promote particle agglomeration. The aggregation of particles can effectively reduce directional migration towards the drain and improve the localized arrangement of particles near the drainage board arrangement of particles near the drainage board， thus relieving the clogging problem of vacuum preloading.

Conclusions and Prospects Flocculants are broadly categorized into inorganic and organic types， each with distinct advantages and limitations. Inorganic flocculants， though relatively cost-effective， require higher dosages and may pose greater environmental risks. Specifically， metal-based flocculants may cause secondary heavy metal pollution. In contrast， organic flocculants achieve superior performance with smaller dosages. Current research primarily focuses on the combined application of inorganic and organic flocculants to leverage the strengths of both. The aggregation of particles by flocculants and the release of free water from sludge by oxidants enhance the permeability of sludge and increase the total drainage volume. Compared to conventional vacuum preloading， chemical agent-assisted vacuum preloading demonstrates marked advantages， including shorter construction periods， lower energy consumption， and improved dewatering effect， as evidenced by key indicators like water content，shear strength， and settlement efficiency. The optimal dosages of chemical agents vary depending on sludge types and the initial water content. In practical projects， the selection and dosage of agents should be optimized based on the specific characteristics of the sludge. At present， chemical agents still pose environmental risks. Future efforts should focus on integrating various types of chemicals to develop efficient， green， and eco-friendly agents. Additionally， it is essential to explore intelligent sludge treatment technologies and equipment to further advance the development and application of chemical agent-assisted vacuum preloading in sludge dewatering.

Keywords：sludge； vacuum preloading； chemical agent； clogging mechanism； dewatering effect