Abstract
Laminated hydrophobic membranes have been proposed as liners for container-based sanitation systems in developing countries. The laminate allows drying of fecal sludge, which might significantly reduce the frequency of container emptying, while containing liquids and solids. While previous laboratory tests demonstrated rapid drying of fecal sludge or water retained in laminates, experiments did not assess the effects of system dimension or scale on performance. In this study fecal sludge drying and water evaporation were evaluated in 3D laminate boxes (decimeter scale) or 3D laminate-lined 40 L and 55 gallon drums (meter scale) that are prototypes of toilet containers for field application. A stagnant film model described fecal sludge drying and water evaporation in the laminate boxes and laminate-lined drums well. The effective diffusion length (λ) for the laminate was fitted in all systems and increased with system dimension and scale: λ increased by a factor of 1.4 from 1D decimeter-scale envelopes to 3D decimeter-scale boxes, and by a factor of 1.3–1.7 from 3D decimeter-scale boxes to 3D meter-scale drums. The longer λ with increasing dimension and scale is likely due to nonuniform temperature and relative humidity in the air outside the laminate and nonuniform temperature within the laminate. Using best-fit λ for the laminate-lined 40 L and 55 gallon drum experiments conducted in a controlled laboratory, drying was predicted for an 11-day field experiment. Although the air temperature and relative humidity varied significantly in the field tests from −1 °C to 26 °C and 35% to 97%, respectively, the stagnant film model predicted drying over the 11-day period reasonably well with total error ≤ 13% using 24-h average air temperature and relative humidity. Drying of fecal sludge in laminate-lined drums in the field might be adequately described with a stagnant film model using daily-average weather conditions, if wind speeds are low.
Highlights
Sanitation systems around the world include conventional sewerage and on-site systems that may include septic tanks, leach pits and pit latrines
Moisture loss from the fecal sludge occurs from the bulk liquid in contact with the laminated membrane as well as vapor transfer from the bulk liquid into the headspace gas phase and through the laminate lid, which is closed during experiments (Fig. 2A)
Building upon previous studies of fecal sludge and water drying in 1D centimeter-scale jar and 1D decimeter-scale envelope experiments, drying tests were conducted in 3D decimeter-scale laminate boxes and 3D meter-scale laminate-lined drums that are prototypes for field deployed laminate-lined toilet containers
Summary
Sanitation systems around the world include conventional sewerage and on-site systems that may include septic tanks, leach pits and pit latrines Some of these on-site systems were traditionally viewed as temporary solutions pending construction of permanent sewers; they serve 2.7 billion people worldwide as the construction of sewers have not kept pace with rapid urban expansion in low and middle-income countries (Strande et al, 2014). Though these on-site facilities provide a safe and private place for defecation, they have had limited success in many poor urban neighborhoods where narrow, irregular streets cause difficulty in removing fecal sludge using suction trucks (Russel et al, 2015). Fecal sludge is defined in this paper to include solid fecal matter, urine, wash water, and some flush water
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