Treatment Performance of Direct Contact Membrane Distillation for Volatile, Semi-Volatile and Non-Volatile Organic Contaminants in Water

Abstract

A laboratory-scale direct contact membrane distillation (DCMD) system was analyzed for treatment performances of a selection of volatile (V), semi-volatile (SV) and non-volatile (NV) organic contaminants, pharmaceuticals and personal care products (PPCPs) and nitrosamines that are of interest to water or wastewater treatment. A group of 32 organics, 8 nitrosamines, and 22 PPCPs were observed with acceptable mass recoveries (> 60%) in the system, with observed recoveries well explained by their lower hydrophobicity (log Kow < 3) and less propensity to sorb to DCMD system components. Due to their low volatility, and consistent with expectations derived from Henry’s law partitioning coefficients (KH; where pKH = -logKH), NV solutes with pKH > 8 were rejected efficiently, with observed rejections of over 90%. Henry’s Law constants estimated at 25°C were not fully predictive of treatment performance during DCMD, indicating that other physical and chemical characteristics contribute to rejection. For example, moderate rejections (i.e. 35% to 69%) were observed for some NV solutes with pKH < 8, as the 50°C feed temperatures increased their apparent volatility in the system. Rejections for SV and V solutes were typically lower, often more variable and sensitive to solute characteristics such as ionizability. In some cases, dissociation constants (pKa) explained higher than expected rejections (e.g. 2-methyl-4,6-dinitrophenol; pKa = 4.31) for ionizable solutes that were non-volatile at system pH. To account for the time dependent characteristics of DCMD batch system, a least square curve fitting modeling approach was used to evaluate the possibility for non-equilibrium, mass transfer limited conditions for SV and V solutes. Permeate fluxes of each contaminant also were developed based on observed DCMD data.