A coupled CFD-evaporation model to estimate drying rates across laminate-lined sanitation systems

ABSTRACT Decentralized sanitation systems such as eco-vapor toilets (EVTs) use hydrophobic laminate-lined containers to collect and dry fecal matter. Removing water in EVTs is critical to their success. While computational fluid dynamics (CFD) modeling of drying can guide EVT designs and site selection, it is expensive. To develop a simpler modeling approach, bench-scale experiments quantified the constant-rate drying period over a wide range of environmental conditions using deionized water as a surrogate for fecal sludge. Then, two modeling approaches were tested using these data: a CFD model, and a simple evapotranspiration model, the Food and Agriculture Organization of the United Nations (FAO) - Penman Monteith (PM) model. The model-predicted drying rates for 29 experiments resulted in a coefficient of efficiency (E, optimal value = 1.0) of 0.47 and −0.05 for the CFD and FAO-PM models, respectively. To reduce the level of effort for a complete CFD model but increase the accuracy of a FAO-PM model alone, a new CFD-Evaporation model is proposed. The FAO-PM and the CFD-Evaporation model applied to 40 L EVTs in an 11-day field test resulted in an E of −1.38 and 0.15, respectively. The CFD-Evaporation model is useful where computational efficiency is more critical than absolute accuracy.