Modeling biosolids drying through a laminated hydrophobic membrane.

The adaptation of the membrane distillation process as a low-cost and sustainable approach to biosolids drying and stabilization is investigated, which may have application in container-based sanitation systems proposed in low-income urban regions. Three-layer laminated, breathable, hydrophobic membranes enclose the biosolids, facilitating drying but preventing transport of contaminants. The membranes used in this process are non-wetting with pore spaces that only allow vapor transport. Water vapor can be expelled due to a moderate vapor pressure gradient. Other constituents, including both particulate and dissolved matter are retained. The permeate, therefore, is expected to be of high purity and pathogen-free. This study presents experimental results showing usable rates of moisture transfer through the laminated hydrophobic membranes with temperature gradients, ΔT = -2 °C, corresponding to the condition that biosolids do not receive external heating in which laminate-enclosed biosolids are 2 °C cooler than outside, as well as conditions that samples are 2 °C and 10 °C warmer than the ambient temperature (ΔT = 2 and 10 °C, respectively). The conditions result in reduction in the moisture content of the laminate-enclosed biosolids from about 97% to 12-30% and the permeate is observed to be free of fecal coliforms, indicator organisms for pathogens. The initial constant-rate drying period is described well with a stagnant film model that accounts for different temperature gradients, laminate surface area, and ambient relative humidity. The proposed model may be used to assess the feasibility of incorporating laminated hydrophobic membranes to enhance biosolids drying in container-based sanitation systems as well as other applications.