Convective drying kinetics of faecal sludge from VIP latrines

In striving to achieve sustainable sanitation, one challenge is to ensure hygienic treatment of faecal sludge from on-site sanitation. Thermal drying is an important treatment step for moisture removal and disinfection. Improved understanding of the drying process is crucial for the proper design of treatment technologies for faecal sludge. In this study, faecal sludge from ventilated improved pit (VIP) latrines from Durban, South Africa, were dried in a convective drying thermobalance by varying the temperature from 40 to 80 °C, the relative humidity from 0 to 25% and the air velocity from 0.3 to 1.2 mm/s. The faecal sludge samples were in the form of a thin layer and pellets with different diameters from 8 to 14 mm. Kinetic parameters were determined from the experimental data, were compared to classical drying models in literature and were then used to develop a correlative drying model. Drying rates ranged between 1 and 40 g/min/m(2), leading to drying times comprised between 100 and 300 min. The drying kinetics increased as temperature was higher, and pellet diameter and relative humidity were lower. Temperature had the greatest influence on the drying kinetics (in both the constant and falling rate periods), followed by the effect of pellet diameter. The drying kinetics were affected in a moderate way by the relative humidity in the constant rate period and part of the falling rate period. The air velocity had a slight effect of drying kinetics during the constant rate period, but this becomes insignificant during the falling rate period. The effective diffusivities increased from 7.81 × 10(−8) to 1.97 × 10(−7) m(2)/s by increasing the temperature from 40° to 80 °C, leading to an activation energy of 23 kJ/mol. These values are typical from those found for wastewater sludge. The sludge exhibited a critical moisture content varying between 2.4 and 3.2 g/g db during drying without a clear trend as a function of the operating conditions and suggested that sludge was composed of considerably more bound moisture than unbound. The experimental data fitted the most closely to the Page model and, based on this, a new model was proposed for the prediction of drying times across the range of explored temperatures and pellet diameters in this study. The results of the proposed model fitted the experimental data with acceptable accuracy, so that the developed model could be employed as an analytical tool for the design, operation and optimisation of drying equipment.