Understanding fecal sludge drying in membrane-lined container-based toilets for developing countries with CFD modeling

Abstract

A new sanitation technology that employs a laminated hydrophobic membrane liner as an integral component of container-based sanitation (CBS) systems was recently proposed for fecal sludge (FS) drying. Previous experimental data were described with a stagnant film model but resulted in an effective diffusive length for the laminate (λ) that increased with system scale and was not a fixed property of the laminate. In this study, computational fluid dynamics (CFD) was used to determine λ for a commercial laminate (eVent® fabric) that is invariant with system scale, to verify the applicability of CFD modeling for describing drying from laminate-lined CBS containers, and to predict the performance of a laminate-lined 40 L toilet in a CBS system for five developing countries. CFD modeling described drying well for experimental systems ranging from centimeter to meter scale using a single, laminate-specific λ, since CFD modeling allowed accurate characterization of the temperature and relative humidity in the vicinity of the laminate. Using λ determined for the eVent laminate, drying of FS from a laminate-lined 40 L toilet was estimated for developing countries selected to cover a range of climatic conditions with an assumed loading rate of 7.8 L FS per day. CFD model predictions showed that the filling time of the laminate-lined 40 L drum increased from 5.1 days with no laminate to 5.3–9.4 days, depending on the location and wind conditions. These modest increases in filling time might be enhanced significantly for alternative container designs that allow more uniform airflow near the laminate enclosure.