## 1 Introduction  
Printing inks are extensively used in publication, packaging, textile printing, and others. In spite of the soaring popularity of e-books and strict regulatory policies over the last decades, the global printing ink market is growing at about 6-7% per year, and more than 4 million tons of ink are consumed annually [1]. In an era marked by growing environmental awareness and sustainability concerns, solvent-based inks are rapidly losing popularity, and water-based inks use water as a carrier, reducing the emission of volatile organic compounds (VOCs). In 2024, the estimated consumption of water-based inks accounted for 25-30% of the overall printing ink market share in China. At the same time, water-based ink wastewater discharge is steadily increasing. Wastewater obtained after cleaning/washing of the laboratory and industrial equipment is characteristically high in both chroma and organic content and is usually difficult to treat biologically. Thus, it will bring severe pollution to ecosystems if not treated satisfactorily [2]. Many challenges remain in finding inexpensive and effective treatment processes for water-based ink wastewater, but there has been little research on the treatment or recycling of this kind of wastewater and its sludge, according to the authors' knowledge. The reported approaches are mainly about pretreatment, including coagulation or flocculation [3, 4], acid precipitation-electrocatalysis [5], chemical oxidation-coagulation [6], adsorption [7], and microfiltration [8], in combination with the biochemical process. Among them, flocculation is a relatively simple and efficient method for removing a substantial portion of the organic content and is especially very good for the removal of color and turbidity. However, flocculation pretreatment of the highly concentrated water-based ink wastewater generally results in a pasty sludge with a suspended solid content of up to 5-9% [3]. Thus, traditional solid-liquid separation methods like air flotation and precipitation are not suitable for sludge separation from pretreated water-based ink wastewater. Actually, the sludge is dewatered directly by mechanical pressure and centrifuge force in many enterprises, in which the dewatering speed and efficiency are very low while the energy cost is high. The obtained sludge still has a large volume and high moisture content (more than 80% after hours of mechanical dewatering). It is known that sludge dewatering is the most important and difficult step in the wastewater treatment process, and it is used to reduce the final sludge volume in order to minimize the cost of sludge transportation and disposal. For the treatment of water-based ink wastewater slurry, figuring out how to improve dewaterability becomes the key issue. The present paper aims to report a simple and effective approach to improving the dewaterability of the coagulation sludge from water-based ink wastewater in order to minimize the volume of sludge. Traditionally, sludge conditioning and modification are the commonly used methods to enhance activated sludge dewaterability, including chemical conditioning [9; 10], freeze-thaw conditioning [11], hydrothermal treatment [12], and electrolysis [13]. The solid content of dewatered sludge would increase after chemical conditioning, while freeze-thawing pretreatment requires special equipment or severe cold regions to reach a low temperature of less than \(-20\)\({}^{\circ}\)C. Compared with chemical conditioning and freeze-thaw conditioning, the hydrothermal method is more frequently applied. Hydrothermal treatment is a thermochemical convention process, which is achieved by applying temperatures of 160-220 \({}^{\circ}\)C under saturated pressure for several hours [14]. Solid-liquid separation properties of sludge can be clearly improved because hydrothermal treatment at high pressure and temperature leads to the disruption of microorganism cells in sludge and the release of the bound water from the sludge [12; 14].