# Exploring Convective Drying Behavior of Hydroxide Sludges Through Micro-Drying Systems
## 1 Introduction  
The generation of sludge is an unavoidable consequence of operations at drinking water treatment plants (DWTPs). In Morocco, the estimated monthly production of sludge derived from potable water treatment reaches approximately 16,000 tons of dry matter (DM). Typically, around 30 g of sludge is produced per liter of water treated [1]. These sludges, commonly referred to as clarification or hydroxide sludges (HSs), are characterized by a low content of biodegradable organic matter and a poor-quality texture [2]. The HSs, which initially exhibit a semi-liquid or \"soupy\" consistency, undergo multiple treatment stages aimed at reducing the water content [3]. These stages involvethickening and dewatering, typically achieved through the addition of substantial amounts of polyelectrolytes. The thickened HS, with a suspended solids concentration of approximately 30 g/L (DM), is further dewatered using sand and gravel filtration systems and subsequently dried through evaporation on drying beds. HS must be either disposed of in regulated landfills or repurposed for other uses [4]. The duration of the drying cycle, which is highly dependent on climatic conditions, particularly precipitation and evaporation rates, typically ranges from 60 to 80 days [1]. The raw water processed during treatment typically contains varying concentrations of suspended solids, including materials such as marl and clay, which are prone to rapid sedimentation [5]. The recovery and storage of these suspended solids, however, present significant technical challenges. In accordance with regulatory frameworks, potable water sludges are generally classified as \"non-hazardous\" or \"ordinary waste\". Nevertheless, the management of HSs has emerged as a critical priority due to the substantial rise in their production, driven by accelerated urbanization and increasingly stringent regulations governing their disposal [2]. Several technologies have been proposed for the treatment and valorization of HSs, with drying standing out as a key technique. This process not only aligns with sustainable development principles but also plays a crucial role in HS management by reducing the mass and volume of the material, consequently lowering costs related to storage, handling, and transportation [6]. The fundamental principle of thermal drying entails the application of energy to a solid-liquid mixture in order to remove the solvent contained within it [7]. The energy required for HS thermal drying can be transferred through mechanisms such as conduction, convection, or radiation. This thermal drying stage, which follows mechanical dewatering, provides significant advantages, particularly in terms of agricultural valorization and potential energy recovery [8]. The target dryness level during the drying process is determined by the specific application for which the HS is intended. Drying operations provide several notable benefits. One of the primary advantages is the significant reduction in both volume and mass through water removal, which directly impacts costs associated with storage, transportation, and handling. Granulated sludges exhibit the added benefit of requiring minimal maintenance during storage. Drying allows HS to be used with minor processing in road filler, cement clinker, quarry reclamation material, landfill sealing, composting co-substrate, built material, among others [8]. The process also stabilizes HS by reducing the possibility of oxidation and fermentation in case of high organic content, mitigating odor, especially when the dryness level exceeds 90%. At such low moisture levels, the decreased water activity inhibits bacterial growth, thus enhancing stability. Moreover, drying contributes to the hygiization of the HS by effectively destroying pathogenic microorganisms when temperatures are sufficiently high (e.g., 30 min at 70 \({}^{\circ}\)C), which is in line with European regulatory directives. Finally, the HS undergoes a transformation from a pasty to a solid texture, facilitating its subsequent handling and spreading.