Solar Thermal Drying Kinetics of Faecal Sludge: Effect of Convection Air Stream Conditions and Type of Sludge

Onsite sanitation offers a sustainable alternative to centralized wastewater treatment; however, effective faecal sludge management is crucial for safe disposal and resource recovery. Among emerging treatment solutions, solar thermal drying holds significant promise to reduce sludge moisture content and enhance handling. Despite this potential, its application remains limited, with important knowledge gaps, particularly concerning the drying kinetics under different environmental and operational conditions. This study aims to fill these gaps by investigating the solar thermal drying behaviour of faecal sludge from ventilated improved pit latrines (VIPs) and urine-diverting dry toilets (UDs), with a specific focus on how air temperature and velocity influence drying performance. A bench-scale solar drying apparatus was used to investigate thin-layer sludge drying kinetics under controlled airflow conditions. The drying experiments were conducted at varying air temperatures (ambient, 40°C, and 80°C) and velocities (0, 0.5, and 1 m/s), where heated air was supplied via an electric resistance heater. Key drying parameters—including drying rate, critical moisture content, effective moisture diffusivity, and activation energy—were determined. Results showed that drying proceeded through a constant-rate period followed by a falling-rate period, with the critical moisture content ranging from 1.41 to 1.78 g/g db. Higher temperatures and airflow reduced the duration of the constant-rate phase, increased drying rates (0.31–0.99 g/g·min·m2), and enhanced moisture diffusivity (4.56 × 10−9 to 1.52 × 10−8 m2/s). Activation energy decreased with increased airflow, suggesting reduced temperature sensitivity. Thermal efficiency ranged from 14.6% to 35.1%, with solar energy contributing 73%–95% of total input. VIP sludge dried faster than UD sludge, which showed signs of surface crusting that limited moisture transfer. This research offers valuable insights into solar drying design and operation, providing scientific evidence to improve faecal sludge treatment strategies in decentralized sanitation systems.