Date of Award
Spring 2023
Document Type
Dissertation
Degree Name
Doctor of Philosophy (PhD)
Department
Chemical and Environmental Engineering (ENAS)
First Advisor
Elimelech, Menachem
Abstract
Water scarcity is one of the biggest threats to humanity, and the global problem is worsening due to population growth, water overuse, pollution, and climate change. Over half of the world’s population suffers from water scarcity at least one month of the year. The problem is further compounded in areas without electricity access. Cape Town’s 2018 “Day Zero” water crisis and Kenya’s 2017 severe drought are indicative of the future. Water conservation alone is not sufficient to meet water demand, especially when power generation is water dependent (e.g., hydropower and fossil fuels). As a result, alternative solutions such as water recycling and treatment of more polluted water sources are becoming increasingly necessary. Desalination is being deployed to treat these water sources while simultaneously meeting stricter water regulations. However, desalination technology can be expensive and energy intensive, which is partly caused by inorganic scaling during the treatment process. The objective of this dissertation is to elucidate desalination pretreatment to mitigate inorganic scaling and investigate a solar thermal-incorporated desalination technology for increasing potable water access globally, regardless of electricity access. One of the critical challenges in desalination processes is controlling inorganic scaling. Mitigating inorganic scaling requires an understanding of why and how nucleation and crystal growth occur. Therefore, this dissertation reviews inorganic scaling mechanisms and current theories and models used to describe inorganic scaling in membrane desalination processes. Two such mechanisms include homogeneous and heterogeneous nucleation, which occur in the bulk and on a surface, respectively. To help guide scaling mitigation research, this dissertation highlights key factors influencing both surface and bulk scaling, such as membrane material and feedwater pH. Scaling characterization techniques are critically analyzed, focusing on strengths and limitations of each in elucidating scaling mechanisms and steering future research addressing a significant desalination technology challenge. A majority of feedwater sources have abundant ubiquitous substances affecting the water’s chemistry and ability to foul and scale a membrane during desalination. Gypsum, or calcium sulfate dihydrate (CaSO4⋅2H2O), is one of the most common and difficult to control scalants in water systems. Two other substances commonly found in freshwater include natural organic matter (NOM) and colloidal particles. In response, the impact of NOM (as Suwannee River humic acid) and colloidal particles with different functional groups on gypsum crystallization is compared. Ultraviolet-visible spectroscopy is used to measure gypsum’s crystallization induction time while scanning electron microscopy (SEM) and X-ray diffraction (XRD) are used for studying the gypsum morphological changes. Humic acid significantly delays induction time and results in a polygon-like shape of gypsum, differing from the characteristic needle-like shape. Colloidal particles, alternatively, can either increase or decrease induction time, but the morphology remains similar to gypsum’s characteristic shape. These findings underscore the importance of determining source water composition before finalizing the pretreatment process. Antiscalants, or scale inhibitors, are the industry standard for controlling inorganic scaling due to their low cost and high efficacy. Despite their known benefits, antiscalants’ mechanisms are not well understood. Therefore, this dissertation reports on antiscalant mechanisms while controlling for the antiscalants’ molecular weight, molarity, and charge. Antiscalants are synthesized by atom transfer radical polymerization (ATRP) to minimize molecular weight dispersity while using selective monomers to control charge. Poly(acrylic acid) (PAA), a negatively charged polymer, has been shown to be effective in mitigating gypsum scale, so PAA was synthesized with three molecular weights and compared based on molarity. The 7,500 Da PAA performs best at all concentrations tested with the 4 mg/L concentration, delaying gypsum induction time the most compared to the other molecular weights. Charge seems to also influence gypsum scaling, with the negatively charge PAA having a superior performance when maintaining molecular weight and molarity. These findings provide insight that can be applied to designing green inhibitors to meet more stringent discharge regulations. Compared to more traditional pressure-driven desalination processes, thermal desalination can be used to treat higher salinity feeds, such as brackish water, desalination brine, industrial wastewater, and reverse osmosis concentrate. This is due to water vapor pressure being a weaker function of salinity compared to osmotic pressure. Membrane distillation (MD), an emerging thermal desalination process, can produce potable water by using thermal energy harvested from the sunlight. Solar-powered MD systems typically include an MD module for desalination, a thermal energy source to heat the feed, an electric power source to drive water circulation, and heat exchangers for heat management. In hybrid solar MD systems, photovoltaic devices are included to supply electrical energy. Hybrid as well as direct and indirect solar MD systems tested at the bench scale up to pilot scale are comprehensively evaluated to identify solar thermal MD’s research needs and determine where solar thermal MD can be applied to meet growing water demand. Photothermal functionalization of the MD membrane, concentration of sunlight, and the MD system design need to be optimized to improve water flux and energy efficiency before solar MD can be widely adopted. This dissertation provides a clearer understanding of inorganic scaling in membrane desalination and its mitigation methods while highlighting a thermal desalination technology that is not limited by salinity or lack of electricity. In doing so, polymeric antiscalants are synthesized; NOM, colloidal particles, and polymeric antiscalants are tested against gypsum scaling; and mechanisms are proposed. Furthermore, the geographic and technological limitations of solar-power MD are analyzed. As a result, this compilation work has elucidated gypsum mitigation methods and guided the study of solar-powered MD, providing insight into improving desalination treatment technology.
Recommended Citation
Rolf, Julianne, "Polymeric Antiscalants for Gypsum Scale Mitigation: Mechanisms and Implication for Membrane Desalination" (2023). Yale Graduate School of Arts and Sciences Dissertations. 868.
https://elischolar.library.yale.edu/gsas_dissertations/868
