Understanding oily wastewater treatment via membrane distillation

Membrane distillation (MD) is an emerging green technology, but very few reports are available on its use for treating oily feeds, despite produced water representing a significant source of oily wastewater. Accordingly, this study was targeted at understanding the primary impediments of oily water MD by systematically investigating the influence of the key components (namely, oil-in-water emulsion, surfactant and salt) in such feeds. When the feed contained all three components typical in produced water, the MD performance severely deteriorated in terms of permeate flux and quality. Interestingly, for feeds containing oil or SDS or NaCl alone, the MD performance was reasonable, which indicates each component on its own did not impact the MD process significantly. Furthermore, the performance was also reasonable when the feed contained oil and NaCl, and improved when the feed contained oil and SDS. The adverse impact on the MD process was thereby traced to the combined presence of SDS and NaCl. Even in the absence of oil, increasing the concentrations of SDS and NaCl in the feed progressively deteriorated the permeate flux and quality. Therefore, to use MD for treating oily feeds necessitates a pre-treatment step to remove or significantly dilute either the surfactant or the salt. More understanding of the interaction between NaCl and SDS via molecular dynamics simulations would be useful to not only provide information on the interaction strength but also on means to circumvent such issues.