Sludge drying through hydrophobic membranes: Mass and heat transfer modeling and practical applications
University of Delaware
One of the major challenges of biowaste management is efficient removal of water from sludges or biosolids, which are generally quite hydrophilic. Also, any water that is removed must be re-treated to remove contaminants that do not partition completely into the solids fraction. To address these issues, the membrane distillation process was adapted for drying and stabilization of sludges. The hydrophobic 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 temperature gradient. Other constituents, including both particulate and dissolved, are retained. The permeate purity, therefore, is expected to be high. This thesis presents data showing usable rates of moisture transfer with modest temperature gradients. In the modeling effort to simulate the moisture transfer from the system, effectiveness and validity of three candidate models were examined: a diffusion equation with flux type boundary conditions; a two-period model, and a stagnant film model. Among these, the stagnant film model is considered as a fundamental modeling framework, because it incorporates all of the factors controlling drying intensity and pattern, and it successfully predicts the mass transfer across the membrane. The resistance associated with the stagnant film was quantified as 0.01 m in the presence of the membrane. The prediction is used to estimate the waste loading that a membrane enclosed pit latrine can handle daily. The provided simulation of the heat transfer accompanied with the mass transfer during the process also allows for calculation and optimization of the energy requirements for the system to operate at the desired rate.