Fluorosis is a major scourge in many countries caused by prolonged consumption of drinking water with high fluoride content found in groundwater resources. Hydroxyapatite (Hap) and its composite forms are excellent biomaterials that recently gained attention as efficient adsorbents, owing to its physical and chemical nature as it can substitute both cationic and anionic complexes present in an aqueous solution in its atomic arrangement. Its biological nature, biocompatibility and biodegradability along with its chemical characteristics such as crystallinity, stability, ion adsorption capability and highly specific catalytic activity make it suitable for a variety of applications especially in water treatment for fluoride removal. This review describes various techniques for synthesis of a wide variety of biogenic, synthetic, composite and modified forms of Hap for application in water defluoridation. Hap derived from natural sources or synthesized using conventional methods, hydrothermal, sol-gel or advanced sonication-cum-precipitation technique varied in terms of its crystallinity, structure, size, etc., which affect the fluoride removal capacity. The advantage and disadvantages of various synthesis methods, process parameters and product characteristics have been compiled, which may help to identify a suitable synthesis method for a desired Hap product for potential application and future perspectives in water treatment.
Static mixers are widely used in various industrial applications to intensify the laminar mixing of non-Newtonian fluids. Non-Newtonian fluids can be categorized into (1) time-independent, (2) time-dependent, and (3) viscoelastic fluids. Computational fluid dynamics studies on the laminar mixing of viscoelastic fluids are very limited due to the complexity in incorporating the multiple relaxation times and the associated stress tensor into the constitutive equations. This review paper provides recommendations for future research studies while summarizing the key research contributions in the field of non-Newtonian fluid mixing using static mixers. This review discusses the different experimental techniques employed such as electrical resistance tomography, magnetic resonance imaging, planar laser-induced fluorescence, and positron emission particle tracking. A comprehensive overview of the mixing fundamentals, fluid chaos, numerical characterization of fluid stretching, development of pressure drop correlations, and derivations of generalized Reynolds number is also provided in this review paper.
To meet the worldwide rapid growth of industrialization and population, the demand for the production of bioethanol as an alternative green biofuel is gaining significant prominence. The bioethanol production process is still considered one of the largest energy-consuming processes and is challenging due to the limited effectiveness of conventional pretreatment processes, saccharification processes, and extreme use of electricity in common fermentation and purification processes. Thus, it became necessary to improve the bioethanol production process through reduced energy requirements. Membrane-based separation technologies have already gained attention due to their reduced energy requirements, investment in lower labor costs, lower space requirements, and wide flexibility in operations. For the selective conversion of biomasses to bioethanol, membrane bioreactors are specifically well suited. Advanced membrane-integrated processes can effectively contribute to different stages of bioethanol production processes, including enzymatic saccharification, concentrating feed solutions for fermentation, improving pretreatment processes, and finally purification processes. Advanced membrane-integrated simultaneous saccharification, filtration, and fermentation strategies consisting of ultrafiltration-based enzyme recycle system with nanofiltration-based high-density cell recycle fermentation system or the combination of high-density cell recycle fermentation system with membrane pervaporation or distillation can definitely contribute to the development of the most efficient and economically sustainable second-generation bioethanol production process.
Anaerobic digestion (AD) is a technology that is gaining popularity because of the need for more renewable energy sources around the world. AD is a complex series of biochemical reactions that ultimately result in the formation of biogas, which is a mixture of methane and carbon dioxide with other trace elements. From large installations to small personal reactors, the underlying basic process is the same, but through research, pretreatments and substrate co-digestion are becoming more popular to enhance biogas production. Reactor design and substrate selection also vary depending on the installation’s location. Biogas cleaning and upgrading help to increase the usability of the gas for multiple applications. The economic viability depends on the location in the world and the available substrate quality and quantity. AD processes rely heavily on government subsidies to stay profitable. In developing countries, AD profitability is not a concern, as this technology provides a way to better human life in these areas. This review presents a detailed look at the AD technology, provides a discussion on the economics of AD, and suggests future studies to enhance the technology.