Improved calcium sulfate recovery from a reverse osmosis retentate using eutectic freeze crystallization

Gypsum Seeding to Prevent Scaling

Eutectic freeze crystallization (EFC) is a novel separation technique that can be applied to treat brine solutions such as reverse osmosis retentates. These are often a mixture of different inorganic solutes. The treatment of calcium sulphate-rich brines using EFC often results in gypsum crystallization before any other species. This results in gypsum scaling on the cooled surfaces of the crystallizer, which is undesirable as it retards heat transfer rates and hence reduces the yield of other products. The aim of this study was to investigate and understand gypsum crystallization and gypsum scaling in the presence of gypsum seeds. Synthetic brine solutions were used in this research because they allowed an in-depth understanding of the gypsum bulk crystallization process and scaling tendency without the complexity of industrial brines. A cooled, U-shaped stainless-steel tube suspended in the saturated solution was employed as the scaling surface. This was because a tube-shaped surface enabled the introduction of a constant temperature cold surface in the saturated solution and most industrial EFC crystallizers are constructed from stainless steel. Gypsum seeding was effective in decreasing the mass of scale formed on the heat transfer surface. The most effective seed loading was 0.25 g/L, which reduced scale growth rate by 43%. Importantly, this seed loading is six times the theoretical critical seed loading. The seeding strategy also increased the gypsum crystallization kinetics in the bulk solution, which resulted in an increase in the mass of gypsum product. These findings are relevant for the operability and control of EFC processes, which suffer from scaling problems. By using an appropriate seeding strategy, two problems can be alleviated. Firstly, scaling on the heat transfer surface is minimised and, secondly, seeding increases the crystallization kinetics in the bulk solution, which is advantageous for product yield and recovery. It was also recommended that the use of silica as a seed material to prevent gypsum scaling should be investigated in future studies.

Opportunities for Building-Scale Urine Diversion and Challenges for Implementation

Urine diversion (i.e., urine source separation) has been proposed as a more sustainable solution for water conversation, nutrient removal and recovery, and pharmaceutical sequestration. As wastewater regulations become more stringent, wastewater treatment plants reach capacity, and water resources become more strained, the benefits of urine diversion become more appealing. By using nonwater urinals and urine-diverting toilets, urine diversion systems seek to collect undiluted human urine for nutrient recovery and pharmaceutical sequestration. Urine is a unique, nutrient-rich waste stream that constitutes an overall low volume of waste entering a wastewater treatment plant. If urine is separated at the building-scale, various technologies can be used to recover nutrients and sequester pharmaceuticals at their most concentrated location. However, the implementation of urine diversion requires a paradigm shift from conventional comingling of wastewater and centralized treatment to source separation and decentralized treatment. This Account proposes a vision for building-scale implementation of urine diversion with the goal of clarifying the opportunities and challenges in this context. The main components of urine, i.e., nitrogen, phosphorus, potassium, and pharmaceuticals, are major drivers for technology development and system implementation. Stepping back, the benefits from water conservation and effects on wastewater treatment are an extension of the system boundary that can impact the sustainability of adjacent systems. However, major challenges have been identified in the literature as hurdles for widespread implementation of urine diversion. Challenges include the comparison of recovering nutrients at the wastewater plant versus at the source, the collection and storage of urine, the ability to recover nutrients and sequester pharmaceuticals, and the overall environmental and economic impacts of urine diversion systems. While these challenges exist, studies have been conducted to address some of the underlying research questions. As more research is conducted, the vision of a seamless urine diversion system with building-wide plumbing and storage comes closer to reality. As such, the application of urine diversion systems will benefit from technology development and research to fill gaps that have been identified. It is important to classify urine diversion systems as a process and not a product. This has implications for the way these systems are evaluated, as their impact on peripheral systems can be of benefit to different stakeholders. In the same light, new research areas, such as cyber-physical systems, reverse logistics, and sustainability transitions, can be applied to urine diversion as approaches for ensuring a robust process for widespread implementation. However, established technologies should be constantly reassessed and enhanced by newer techniques. For example, membrane distillation, eutectic freeze concentration, and solar evaporation should be considered for nutrient recovery and volume reduction because they offer benefits over conventional technologies. Finally, the human behavior component of urine diversion cannot be ignored, as negative user acceptance and improper maintenance of these systems can have a detrimental impact on their future implementation.