The high water content of sludge increases the costs of subsequent treatment as well as environmental risks. An innovative process of low-temperature combined with polyacrylamide (PAM) under near-neutral conditions (heat-PAM) was proposed for enhancing sludge dewatering in a simple and practical way. At the pH of 6.0 and temperature of 85°C, with PAM dosage of 10.0 mg/g dry solid (DS), the water content and capillary suction time (CST) of treated sludge decreased from 76.8 % and 27.5 s to 68.9 % and 16.6 s, respectively. The near neutral pH is conducive to further resource utilization of sludge. This heat-PAM treatment enabled the transformation of hydrophilic fraction within tightly bound extracellular polymeric substances (TB-EPS) to soluble EPS (S-EPS), the lysis of the sludge cells, and the release of bound water. The bound water content was reduced from 2.4 g/g DS to 0.8 g/g DS. Furthermore, molecular characterizations of EPS by X-ray photoelectron spectrometry (XPS) clarified the decomposition of hydrophilic functional groups by the heat-PAM treatment. Zeta potential and particle size indicated that PAM caused a re-flocculation. Changes in the distribution of microbial community within treated sludge resulted in a decrease of the proportion of certain antibiotic resistance genes (ARGs). Meanwhile, the contents of typical heavy metals (Cr, Cu, Zn) in conditioned sludge decreased significantly. This study presents a simple and effective technology for enhancing sludge dewatering and removing pollutants from sludge.

Waste activated sludge is a byproduct of wastewater treatment with extremely high water content (usually > 99 %), which limits its subsequent treatment and disposal [1], [2]. The cost of sludge treatment represents a substantial portion of the overall expenses for wastewater treatment plants (WWTPs) [3]. It is known that sludge contains many harmful substances, such as heavy metals and antibiotic resistance genes (ARGs) [4], [5]. The content of antibiotics in sewage sludge varies greatly, ranging from μg/kg to mg/kg [4]. These pose a potential threat to human health and greatly limit the resource utilization of residual sludge. Therefore, it is necessary to remove these harmful substances during sludge dewatering.

Extracellular polymeric substances (EPS) are negatively charged hydrophilic polymers found in sludge that determine sludge dewatering performance [3]. EPS are primarily composed of proteins (PN) and polysaccharides (PS), in addition to humic substances and nucleic acids [6]. According to the degree of cell binding, EPS can be classified as soluble EPS (S-EPS), loosely bound EPS (LB-EPS), and tightly bound EPS (TB-EPS) [7], [8]. The hydrophilicity of EPS allows them to bind to large amounts of water, whereas the electronegativity of EPS allows for the stable dispersion of sludge in wastewater systems [9]. The loose structure of the sludge floc makes it difficult to discharge attached water, resulting in higher sludge water content. Therefore, sludge is usually pretreated before dewatering to achieve better dewatering performance [10].

Temperature is a main factor affecting sludge dewatering performance. Studies have shown that significant enhancements in sludge dewatering capability are only observed when temperatures exceed 180°C [11]. However, high-temperature treatment releases organic matter from the sludge into the filtrate. This not only reduces the utilization value of the dewatered sludge cake, but also results in extremely high soluble chemical oxygen demand (SCOD) and ammonia nitrogen concentrations in the filtrate. Consequently, the subsequent treatment process becomes more complex [12]. In addition, high equipment requirements and high energy consumption increase the processing cost [12], [13]. Hence, it is necessary to develop low-temperature sludge treatment technology to reduce costs. The low temperature heat treatment can break the sludge flocs structure and decompose EPS. However, it can increase the surface area of the sludge, thus deteriorating the sludge dewaterability [9]. Previous studies have explored the combination of low temperatures and oxidants to synergistically enhance sludge dewatering performance. It has been reported that sodium persulfate can reduce water content of sludge by 12.2 % at 80℃ [14]. Calcium peroxide combined with low temperature can reduce the water content from 79.9 % to 69.2 % under acidic conditions [9]. However, there are potential risks associated with the storage and use of oxidants, and improper handling of them can cause significant harm to human health and the environment. Moreover, oxidant activation typically requires acidic conditions or involves inherently acidic compounds, resulting in highly acidic filtrate after treatment. This acidic filtrate not only causes corrosion hazards to treatment equipment, but also necessitates alkali dosing for neutralization, thereby increasing downstream treatment costs. Therefore, how to enhance sludge dewatering by low temperature treatment without the use of oxidants is worthy of attention.

Cationic flocculants can eliminate the electrostatic repulsion between sludge particles and destroy the sludge suspension system, thereby enhancing sludge dewatering [3]. Polyacrylamide (PAM), a commonly used cationic flocculant, is widely applied in WWTPs due to its low cost and strong flocculation [15]. During sludge flocculation process, PAM act as a connecting bridge between the small and large sludge flocs through charge neutralization and bridging, thereby increasing the rate of sludge filtration [16], [17]. However, if only PAM is used, the EPS structure will not be destroyed and the bound water will remain in the sludge, resulting in persistently high water content [18].

The combination of PAM and low temperature can make up for the shortcomings of low-temperature treatment and PAM in strengthening sludge dewatering. The combined technology also avoids the use of oxidants, thereby minimizing the environmental risks associated with chemical use. In addition, heat treatment can destroy sludge cells and change the microbial community structure, which has the potential to remove ARGs from sludge [11]. However, there remains a research gap in the removal of ARGs by heat treatment combined with PAM in the process of enhanced sludge dewatering.

This study aims to explore the potential improvement in sludge dewatering using low-temperature combined PAM under nearly neutral conditions (heat-PAM). The capillary suction time (CST) and water content were employed as parameters to assess dewatering performance. The primary objectives of this study are as follows: (1) investigate the effect of heat-PAM process on sludge dewaterability; (2) assess the variations in typical heavy metals contents and the relative abundance of some ARGs in sludge after combined conditioning; (3) explore the relevant mechanisms of enhancing dewatering performance of sludge, and evaluate the alterations in microbial community structure to reveal the potential mechanisms for removing ARGs from sludge.