Breathable membrane enclosure for fecal sludge drying and stabilization
Date
2017
Authors
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Journal ISSN
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Publisher
University of Delaware
Abstract
Drying of fecal matter enclosed in a breathable, hydrophobic membrane laminate was investigated for the potential application of breathable laminates in decentralized container-based sanitation system. Moisture loss from the membrane-enclosed fecal sludge was studied using membrane “envelopes” filled with fecal sludge collected from volunteers. Drying test with a new membrane envelope resulted in 71.2% mass reduction over a period of 7 days with a linear average moisture flux of 0.73 g/day-cm2 observed in the first 12-hour period. Slight decrease in the sludge drying rates was observed over five “reuses” of the same membrane envelope. The experimental data were fitted to the stagnant film model to obtain mass transfer resistance value () of 7.23 x 10-3 for fecal sludge. Stagnant film model was used to predict drying rates of membrane-enclosed fecal sludge in ten populated cities of developing nations around the world. Based on a loading rate of 10 L/day into a 55-gallon collection container, the predicted drying rates ranges from 2.4 - 7.5 L/day. The filling time of the membrane laminate-lined container decreases due to in-situ drying, resulting in longer operation time and less frequent emptying of the container. ☐ The drying rates observed with a “membrane box” demonstrate different behavior for fecal sludge and deionized water. Vapor flux remained constant with decreasing water level in the deionized water box, while drying rates of fecal sludge decreased with decreasing wetted surface area. This difference was attributed to capillary rise in the backing fabric of the membrane laminate for the water boxes. The experimental data were fitted to the stagnant film model to obtain mass transfer resistance value () of 1.16 x 10-2 and 1.04 x 10-2 for fecal sludge and water, respectively. Based on the observed moisture loss in water filled laminate-lined 40-L and 55-gallon drums, a lambda value of 1.38 x 10-2 and 1.73 x 10-2 was obtained using the stagnant film model. Higher mass transfer resistance values of the drums compare to the membrane boxes were attributed to the presence of air pockets between the container wall and membrane surface. Using the lambda values obtained from the control room experiments, the stagnant film model predicted the drying rate from laminate-lined drums exposed to real-time weather changes within 20% of measured flux. ☐ Drying of fecal sludge enclosed in a pilot-scale laminate-lined 55-gallon and 40 L drums (referred as Eco-Vapor toilets) were investigated. These Eco-Vapor toilets (EVTs) were constructed and pilot tested in four households in urban slums of Kanpur, India over a period of two years (February 2015-December 2016). Loading rates of these toilets were limited to 5 L/day by restricting the use to four per day with 1 L wash-water/use. Moisture loss of 1.20 L/day and 1.21 L/day were observed in laminate-lined 55-gallon drums and this in situ drying decreased disposal frequency by 12 days. Stagnant film model using meteorological data closely predicted observed mass flux within 10% error. Modified EVTs with 40 L laminate-lined drums and waste segregation were constructed in Year 2 of the field trials. These smaller, more compact drums with waste separation presented the opportunity to increase the number of uses per day to match the user household capacity. Waste segregation resulted in higher total solids inside the drum, which reduced the yearly average moisture flux to 0.40 kg/day. A qualitative study aimed at the toilet user and their respective communities was conducted to assess the perceptions of an urban slum dweller in adapting the new Eco-Vapor toilets. Self-pride, personal safety, convenience of a household toilet, saving of time and medical expenses are some of the documented positive influences for installing a toilet. Additionally, the problems with the existing sanitation infrastructure such as dilapidated infrastructure, lack of maintenance, high user fee, long wait times and insufficient access to water were highlighted in these urban slum areas.