In-situ drying of faecal sludge in breathable membrane-lined collection containers

Abstract

Drying of faecal sludge enclosed in a breathable, hydrophobic membrane laminate was investigated for the potential application of breathable membranes in decentralized container-based sanitation systems for developing nations. Moisture loss from the membrane-enclosed faecal sludge was studied using membrane ‘envelopes’ filled with faecal sludge collected from random volunteers. A drying test with a new membrane envelope resulted in 71.2% mass reduction over a period of 7 days with an average moisture flux of 0.73 g/day-cm2. Slight decrease in the sludge drying rates was observed over five reuses of the same membrane envelope. A stagnant film model was used to predict drying rates of membrane-enclosed faecal sludge in ten developing countries with high urban populations. Based on a loading rate of 15 L/day into a 200-L (55-gallon) collection container, the predicted drying rates range from 7.1 to 12.4 L/day. The filling time of the membrane-lined container decreased due to in-situ drying, resulting in longer operation time and less frequent emptying of the container.

Drying of fecal sludge in 3D laminate enclosures for urban waste management

“…One hypothesis tested in this study is that the stagnant film model adequately describes water vapor transport through laminated hydrophobic membranes lining 3D enclosures at intermediate scale, such as the boxes. While this model described water evaporation and drying of biosolids and fecal sludge when vapor transport occurred in 1D (Sherwood et al, 1975; Marzooghi et al, 2017; Bakhshayesh et al, 2018; Saxena et al, 2019), in 3D enclosures at larger scale nonuniform gradients of temperature and relative humidity inside and outside the enclosure challenge the utility of this modeling approach. The stagnant film model does not account for air flow around the box, nor the buildup of water vapor in the external air along the downwind sides of the box.…”

“…Bakhshayesh et al (2018) examined repeated 1D drying of fecal sludge from centimeter-scale jars covered with eVent laminate (Laminate P4PS4039-3L, eVent fabrics, Lee's Summit, MO, USA) and found no significant reduction in drying performance over five drying cycles, if a moderate brushing/rinsing cleaning procedure was followed between use. Saxena et al (2019) proposed a container-based toilet system with the eVent laminate lining containers that allowed drying of fecal sludge during collection, thus reducing the frequency of container replacement and sludge disposal. Applying the stagnant film model with parameters determined from 1D decimeter-scale envelope experiments, Saxena et al (2019) predicted moisture loss from laminate-lined containers in 10 representative developing countries.…”

“…Saxena et al (2019) proposed a container-based toilet system with the eVent laminate lining containers that allowed drying of fecal sludge during collection, thus reducing the frequency of container replacement and sludge disposal. Applying the stagnant film model with parameters determined from 1D decimeter-scale envelope experiments, Saxena et al (2019) predicted moisture loss from laminate-lined containers in 10 representative developing countries. Important limitations of these previous studies is that all experiments were small-scale, with laminates on the order of centimeter to decimeter scales; drying occurred under constant, controlled environmental conditions; and designs were only evaluated where drying occurred in one dimension.…”

“…Important limitations of these previous studies is that all experiments were small-scale, with laminates on the order of centimeter to decimeter scales; drying occurred under constant, controlled environmental conditions; and designs were only evaluated where drying occurred in one dimension. Thus, the predictions of fecal sludge drying from laminate-lined containers in developing countries by Saxena et al (2019) have a considerable degree of uncertainty.…”

“…The effective diffusion length, λ (m), is defined asλ=italicδτ/εwhere δ is the laminate thickness (m), τ is the dimensionless tortuosity, and ε is the porosity. The effective diffusion length is a measure of mass transfer resistance through the breathable laminate and was previously fitted to experimental data for 1D drying in centimeter-scale laminate jars (Bakhshayesh et al, 2018) and decimeter-scale envelopes (Saxena et al, 2019). In this study, the effective diffusion length was fitted to data from 3D laminate enclosures at decimeter and meter scales.…”