Student assignments

Background

Cake filtration is an old and established separation method for separating solids from fluids, with wide ranging applications in industry [1]. Common applications are found in chemical, petrochemical, pharmaceutical, mining and food industries [2]. Besides the obvious application of separations of solids from suspensions (e.g. recycling of catalysts), also the adsorption of compounds (e.g. discolouration of olive oil by bleaching earth) is also an important use case for cake filtration. The combined removal of small particulate matter through filtration and unwanted chemicals due to adsorption leads to an efficient integrated process step.

Although the basic concepts of cake filtration are easily understood, it is a complex process with many interconnected variables. Still over the long lifespan of this process many developments have been made, both in filter engineering and understanding of the process [2]. However industrial design is still complex, partly due to a large variety of feed stocks and applications. Therefore there is a lot to study from both a chemical engineering and physics perspective.

One of such developments is the Cricketfilter® element developed by Amafilter. It is an evolution of the classic vertical ‘candle filter’ element, in which the internal volume is reduced to maximise the available filtration area in a given vessel volume.

Master thesis assignment

In this assignment the focus is on describing the cake filtration process in a vessel with a Cricketfilter® element. Investigating the particle properties (e.g. size, shape and charge) and relating this to the results obtained from filtration cycles leads to improved understanding of the process and possible prediction of future filtration performance. In order to do this a coupling of the possible interactions between the particles in different conditions (repulsion and agglomeration as can be described by DLVO or physical linkage or hindrance due to particle strucure) to the resulting filtration performance and cake structure will need to be undertaken. Once an understanding of the build-up of cake material on the Cricketfilter® element has been established, possible use cases such as adsorption of micropollutants in the solid material of a cake layer can be investigated. Finally the dynamic cake filtration process also leads itself well to an analytical/numerical approach to gain further insights into the development of the cake layer under varying conditions.

The following aspects are of high interest:

• Experimental investigation of the operation of the cake filtration process on the Cricketfilter® element, using various materials (and combinations thereof) and operating conditions
• Determining relations between the properties of the materials used, their filtration performance and finally the resulting cakes obtained.
• An analytical/numerical description of the cake/cake build-up
• Experimental investigation into the use of a cake layer for the adsorption of contaminants from an aqueous phase

If you are interested in this assignment, please contact Jurgen Roman (j.b.roman@utwente.nl).

[1] Tien, C. (2002). Cake filtration research—a personal view. Powder Technology, 127(1), 1–8. https://doi.org/10.1016/S0032-5910(02)00063-3
[2] Khean, T. S. (2003). Studies in filter cake characterisation and modelling. https://core.ac.uk/works/9093340

In our everyday lives we frequently use products which contain fluorinated chemicals (non-stick cookware, cosmetic products, water resistant clothing, ect.). In order to produce these fluorinated coatings the industry uses poly- or perfluorinated alkyl substances (PFAS) in their processes. Both the waste streams from these production processes and excretion from these products themselves leads to these PFAS entering our water supplies. Many of the problematic molecules in this class of compounds (containing >9000 different compounds) contain a charged head group giving rise to high solubilities in water combined with a very stable carbon-fluorine tail. The strength of the fluorine-carbon bond means that these compounds are non-biodegradable and they will therefore ‘almost’ indefinitely build up inside of the water system. For this reason these compounds are also often referred to as the ‘forever chemicals’. 

In recent years there is increasing concern that the ubiquitous presence of these PFAS in water resources is not addressed by current water treatment facilities and therefore they end up in drinking water. Currently the only widely employed method for removal of PFAS from water is adsorption onto activated carbons [1],  however the use of activated carbons seems to be unsustainable from both a economical perspective and a environmental perspective. In the Netherlands the concern about these compounds has consistently shown up in the news over the past couple of years, although many regions in the USA [2] and China [3] are substantially more severely impacted. 

In this project you will use bio-based materials of different origins and test their ability to adsorb PFAS from water. Both batch testing and flow through testing will be employed to asses adsorption capacity and kinetics at environmentally relevant concentrations. Furthermore you will need to characterise the adsorbents to be able to make a fair comparison. Fitting of adsorption models, statistical analysis and choices in data presentation will be instrumental to make your results both relevant and meaningful. This is a very active field of study, so your view on what practises are correct and which are less relevant is very valued. The assignment is suitable for students of universities (both bachelors and masters level) and students of universities of applied sciences.

This assignment is part of a project to develop a new process to treat water resources plagued by PFAS to make drinking water that is safe for consumption. The data you collect will be directly used for the future direction of the project!

If you are interested in this assignment, please contact Jurgen Roman (j.b.roman@utwente.nl).

[1] Xiao, X., Ulrich, B. A., Chen, B., & Higgins, C. P. (2017). Sorption of Poly- and Perfluoroalkyl Substances (PFASs) Relevant to Aqueous Film-Forming Foam (AFFF)-Impacted Groundwater by Biochars and Activated Carbon. Environmental Science and Technology, 51(11), 6342–6351. https://doi.org/10.1021/ACS.EST.7B00970

[2] Johnson, C. D. (2022). Per- and Polyfluoroalkyl Substances: A Preliminary Evaluation of Groundwater Contamination in the Western States. https://www.osti.gov/biblio/1879543

[3] Liu, L., Qu, Y., Huang, J., & Weber, R. (2021). Per- and polyfluoroalkyl substances (PFASs) in Chinese drinking water: risk assessment and geographical distribution. Environmental Sciences Europe, 33(1), 1–12. https://doi.org/10.1186/s12302-020-00425-3

The discharge of saline waste water streams into surface waters, seas and oceans becomes increasingly difficult because of proven ecological impacts on aquatic life [1]. Especially in Asia (China and India) strong restrictions regarding the discharge of these streams apply and increasingly stricter regulations are expected for Europe, the Middle East and the Americas [1,2]. This means that zero liquid discharge (ZLD) concepts, where saline streams are (further) concentrated and subsequently sent to a salt crystallizer to eventually produce pure water and crystalline salts are increasingly important. Evaporation technologies are commonly used to perform the final concentration step in ZLD concepts. However, even though steam re-use in multi-effect distillation or electricity use in mechanical vapor compression units reduces the energy consumption in these evaporation processes, energy consumption and associated costs are still considered high. As an alternative for evaporation technologies, membrane-based technologies are studied. These technologies include reverse osmosis (RO), which is commercially used for the production of drinking water from brackish water or sea water. Using RO, these salt containing streams can be concentrated up to some 7 %w salt [1,3]. Producing retentates with even higher salt concentrations would lead to high osmotic pressures and consequently even higher operating pressures to maintain sufficient driving force for the concentration process [4]. Even with ultra-high pressure reverse osmosis it is not possible to reach saturated salt solutions suitable as feedstock for the salt crystallization process [5].

However, for specific RO processes, such as osmotically assisted RO (OARO) [5] and loose RO [6,7], the difference between the osmotic pressure at the concentrate and permeate side is limited and a salt saturated concentrate can be produced [5]. In the last couple of years the interest in these technologies has grown considerably and a high number of publications related to these technologies have appeared. Some of these publications even show a global unit lay-out and an evaluation of the required energy consumption. However, for loose RO hardly any attention is paid to the maximum allowable salt retention as function of the retentate salt concentration to meet the maximum acceptable osmotic pressure criterion and the technological and economic feasibility of this retention. Furthermore, a more detailed design, featuring the optimal module size, the optimal number of modules per pressure housing, the number of pressure-housing operated in parallel and/or in series for different feed capacities has not (yet) been reported and a proper selection regarding the optimal design type (Christmas tree or cross-flow operation) has not been shown.

The generation of this missing information will be the subject of this MSc project. The required relation between salt retention and salt concentration for typical RO membranes will be obtained from open literature or, if not available, generated through a limited amount of experiments. When typical RO membranes show too high retention, adaptation of the tight thin film composite layer can be achieved using NaOH or chlorine containing solutions to open-up the RO membranes thereby creating loose RO membranes, which will be tighter than the commercially available nanofiltration membranes. The majority of the project will deal with the modelling and the design of the unit for different flow capacities, salt inlet concentrations, salt compositions, and membrane characteristics. Optimization will be done based on an estimation of required capital investment and energy consumption. The outcome of the project will give guidelines for required membrane development and a first idea about the technological and economic feasibility of loose RO for the concentration of salty waste streams.

Options for an MSc assignment

• Evaluation of loose RO for zero liquid discharge (ZLD) starting from sea water (process design).
• Evaluation of the feasibility and availability of loose RO membranes for concentration of 7 %w NaCl solution up to saturation.

Interested? Please contact Sander Haase or Gerrald Bargeman.

References

1. Tong, T., & Elimelech, M. (2016). The global rise of zero liquid discharge for wastewater management: drivers, technologies, and future directions. Environmental science & technology, 50(13), 6846-6855.
2. Wang, Z., Deshmukh, A., Du, Y., & Elimelech, M. (2020). Minimal and zero liquid discharge with reverse osmosis using low-salt-rejection membranes. Water research, 170, 115317.
3. Peters, C. D., & Hankins, N. P. (2019). Osmotically assisted reverse osmosis (OARO): Five approaches to dewatering saline brines using pressure-driven membrane processes. Desalination, 458, 1-13.
4. Davenport, D. M., Deshmukh, A., Werber, J. R., & Elimelech, M. (2018). High-pressure reverse osmosis for energy-efficient hypersaline brine desalination: current status, design considerations, and research needs. Environmental Science & Technology Letters, 5(8), 467-475
5. Bargeman, G. (2022). Creating saturated sodium chloride solutions through osmotically assisted reverse osmosis. Separation and Purification Technology, 293, 121113.
6. Wang, Z., Feng, D., Chen, Y., He, D., & Elimelech, M. (2021). Comparison of energy consumption of osmotically assisted reverse osmosis and low-salt-rejection reverse osmosis for brine management. Environmental Science & Technology, 55(15), 10714-10723.
7. Bargeman, G. (2023). Maximum allowable retention for low-salt-rejection reverse osmosis membranes and its effect on concentrating undersaturated NaCl solutions to saturation. Separation and Purification Technology, 123854.