Convective Drying Kinetics of Faecal Sludge from Vip Latrines

Abstract

In striving to achieve sustainable sanitation, one challenge is to ensure hygienic disposal of faecal sludge from on-site sanitation facilities.  In order to safely dispose of the sludge, drying can be carried out to reduce the volume and mass of the waste for transportation, to deactivate pathogens making the sludge safer to handle and reuse, and finally to increase the calorific value of the sludge for potential fuel use. Improved understanding of the drying process and kinetics are crucial for the design of treatment facilities for faecal sludge and the quickest way to do this is to perform drying analysis of faecal sludge in laboratory small scale set-up. In this study, faecal sludge from ventilated improved pit (VIP) latrines from the Durban area in South Africa were dried in a convective drying thermobalance by varying the drying temperature from 40 to 80oC, the relative humidity from 0 to 25% and the velocity of the supplied air from 0.3 to 1.2 mm/s. The faecal sludge samples were in the form of a thin layer or pellets with different diameters from 8 to 14 mm.  Drying curves were plotted from the experimental data and used to determine the drying rates, critical moisture content and effective diffusivities of moisture through the faecal sludge with their respective activation energies. The drying curves data were then fitted to various common models from literature, and a faecal sludge drying model was developed from this analysis. Drying rates were in the range between 1 and 40 g/min/m2, increasing as temperature and air velocity were increased, and pellet diameter and relative humidity decreased.  Temperature and pellet diameter had the greatest influence on the drying rate, whereas the drying kinetics were affected in a moderate way by the relative humidity, and insignificantly by the air velocity under the explored conditions. The effective diffusivities increased from 7.81 x 10-8 to 1.97 x 10-7 m2 /s by increasing temperature from 40° to 80°C, with an activation energy of humidity. The experimental data fitted most closely to the Page model and, based on the latter, a new model was proposed for the prediction of drying times across a range of temperatures and pellet diameters. 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.