Organic fouling of desalination membranes

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Mechanical Engineering, 2017.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Cataloged from student-submitted PDF version of thesis.Includes bibliographical references (pages 211-224).Energy-ecient desalination and water reuse are necessary to ensure universal access to clean water. Reverse osmosis (RO) is the most ecient desalination process for almost any water source, but it is susceptible to membrane fouling, which can reduce product water quality and raise energy consumption. Fouling can be reduced through (energy-intensive) pretreatment, delayed by membrane coatings, and partially reversed by cleaning. However, poor understanding of fouling physics hinders our ability to predict fouling or design for fouling resistance. Better models of fouling are needed to improve the RO process and provide sustainable sources of desalinated or recycled water to water-scarce communities. Through experiments and modeling, this thesis compares several desalination systems, quantifies the effect of pressure on fouling, and elucidates mechanisms of foulant removal. An experimental apparatus was created to simulate operating conditions in full-scale RO, forward osmosis (FO), and membrane distillation (MD) desalination systems and compare the fouling behavior of these processes under identical hydro-dynamic conditions. In the FO configuration, both uid streams could be pressurized to experimentally isolate the effects of pressure from other operating conditions that affect fouling. A window in the membrane module allowed in situ visualization of membrane fouling and cleaning at pressures as high as 69 bar. Experiments were complemented by the development of physics-based models that predict the eect of hydraulic pressure on foulant layer properties and ux decline and also enable the calculation of foulant layer thickness from measured flux. The findings provide new insight into the relative fouling propensity of membrane desalination systems, the factors influencing ux decline, and the mechanisms of foulant removal. Experiments and modeling show that, although flux decline is slower in FO than in RO, the FO membrane accumulates a thicker foulant layer. Furthermore, FO fouling trials at elevated pressure reveal that fouling behavior is not adversely affected by high hydraulic pressure. Despite this, low operating temperature and unfavorable surface chemistry cause RO to be more susceptible to organic fouling than MD and more susceptible to inorganic fouling than FO. However, neither FO nor MD is immune to fouling: FO flux declined as much as RO ux in the presence of alginate fouling, and MD exhibited rapid ux decline as a result of inorganic fouling. Finally, in situ visualization revealed that osmotic backwashing causes the foulant layer to swell, buckle, and detach in large pieces from both FO and RO membranes, regardless of operating pressure. These findings guide desalination process selection, membrane design, and cleaning protocol development to reduce the energy consumption associated with membrane fouling in desalination.by Emily Winona Tow.Ph. D