Nadimi, Maedeh (2024) Strategies for Effective Membrane Distillation Operation during Treatment of High Strength Industrial Wastewater. PhD thesis, Victoria University.

Abstract

Membrane distillation (MD) is a promising technology for producing high-quality distillate from highly concentrated industrial process wastewater streams containing dyes (e.g. from textile industries, paper and pulp industries, and pharmaceutical industries). Despite its potential, MD faces significant challenges, including organic fouling and feed liquid breakthrough (wetting), exacerbated by the presence of the high-strength dye solutions containing substantial amounts of salts. These challenges can be addressed by either oxidation of the dyes to increase their hydrophilicity or enhancing the hydrophilicity of the membrane surface. This research, therefore, explores these two primary methods to overcome these MD performance challenges involving the integration of sulphate radical-based advanced oxidation processes (SR-AOPs) and the development of hydrophilic surface modifications on commercial MD membrane. The initial focus of this research is the integration of SR-AOPs with MD to enhance system performance. Oxone was applied as an oxidant under various conditions, including without pH adjustments, with pH adjusted to approximately 6.7, and considering the isolated effects of acidic conditions induced by hydrochloric acid. A key observation was that while the addition of Oxone delays the onset of wetting, it fails to improve MD flux or reduce fouling. This limited effect can be attributed to the high salinity and high organic concentrations in the feed, leading to the identification of a likely effect known as salting-out. Further testing by increasing Oxone dosing or reducing the organic load could potentially enhance the degradation of MB and potentially improve MD performance even at higher salt concentrations, but the dose amounts became impractically high. In another approach to address dye fouling and wetting challenges, this study applied hydrophilic surface modifications using polydopamine (PDA). This was guided by literature as a means to increase hydrophilicity and decreasing foulant adhesion. The success of PDA modification was confirmed by water contact angle and identification of functional groups by FTIR analysis. Despite the confirmed functional surface improvement, MD testing found wetting was only delayed and not prevented. This corroborated with the Oxone testing results that the challenge is further complicated by the diverse interactions among ions, water molecules, and nonelectrolytes (e.g. organic material) in high salinity environments. This can outweigh any changes (even major changes) to either solute or membrane surface chemistry, giving motivation towards an investigation into salting-out effect. The dedication towards understanding the science behind the salting-out effect was therefore due to its demonstrated significant role in MD performance, acting to decrease the solubility of organics and causing them to precipitate in high-salt solutions. This phenomenon has not previously been investigated within the context of MD. The influence of salting-out was quantitatively assessed by measuring the Setschenow/salting-out constant for a specific MD foulant. Another key finding of this research is the direct correlation established between MD flux, the solubility of MB, and the mass of MB aggregates. Additionally, the identification of a critical mass threshold for these aggregates that triggers wetting provided a new and practical means to predict the impact of salting-out on both flux behaviour and wetting in MD processes. In conclusion, this study confirmed that SR-AOPs and hydrophilic modifications provide limited mitigation against wetting while concentrating high salinity industry dye wastes where MD is likely to be practically applied. Salting-out was confirmed to be a dominant effect influencing MD performance. Future work should integrate reactive processes with MD using salting-out theory and explore its applicability to other compounds and membranes. These efforts aim to advance technologies for economically viable zero liquid discharge (ZLD) of highly concentrated wastewaters, enhancing sustainability and efficacy across various industries.

Search Google Scholar

Repository staff login