Drying of fresh faeces and faecal sludge is a productive step in sanitation and waste treatment, and an improved understanding of fresh faeces and faecal sludge drying is critical to the deployment of decentralized faecal sludge management services. This work determines 1) the moisture sorption isotherm for fresh faeces and 2) proposes the use of the Modified Halsey model for predicting the water activity and calculating the heat of sorption for fresh faeces. In addition to informing drying process design, the sorption isotherm can be used to predict microbial activity, which could potentially improve the management of faeces and faecal sludge from a public health perspective. These data in turn will be used to promote universal access to sanitation.
Drying is a complex, multi-equilibrium process, and elegant models have been developed to summarize the process. There is no published literature for key drying characteristics or a widely-accepted model that estimates these characteristics for fresh faeces. To measure these key characteristics, static gravimetric analysis was performed over variable humidity and temperature to produce experimental isotherm data. The experimental isotherm data indicates a Type 2 isotherm characterized by a sigmoid shape with two inflection points. This sigmoidal sorption isotherm describes both multilayers of water on the surface of a material and water found within the capillaries and adsorbed within the material. Models were fitted to the experimental data by 1) estimating parameters for each temperature considered, and then 2) estimating parameters with the use of modified model versions that consider temperature dependence. The BET model fit best in the first phase of model fitting, and the Modified Halsey model fit best in the second. Goodness-of-fit was evaluated by a statistical analysis of the root-mean-square error (RMSE). The Modified Halsey model was then used to calculate the heat of sorption.