Nanofiltration of Greywater to remove Micropollutants
Sam Rutten is a PhD student in the department Membrane Science & Technology. (Co)Promotors are prof.dr.ir. W.M. de Vos and dr.ir. J. de Grooth from the faculty of Science & Technology and dr.ir. L. Hernández Leal.
Global freshwater resources are currently experiencing increasingly high stress due to extreme weather events brought along by climate change and the ever-growing global population. To protect these natural resources while simultaneously meeting the growing global water demand, local solutions, such as the reclamation of domestically produced wastewater, have gained interest. When considering wastewater reclamation, implementing source-separated sanitation concepts, where toilet water (blackwater) is separately collected from all other sources of domestic water (greywater), can enhance the ease of recovery of nutrients, energy, and water. Effective biological greywater treatment systems have been developed and implemented in the past decades, opening the door to the potential reuse of this stream has limited the full-scale adoption of greywater treatment plants for water reclamation. Micropollutants consist of a large group of natural and anthropogenic substances with a molecular weights up to 800 g.mol-1, such as pharmaceuticals like diclofenac and personal care products like galaxolide. These substances are found at trace concentration (ng.L-1 - µg.L-1) in wastewater and could have potential detrimental effects on the environment when discharged. Therefore, advanced post-treatment solutions are required to reduce the continuous discharge of micropollutants by treatment plants and limit the risks posed by these substances during reuse. One promising post-treatment technology is the use of hollow fiber nanofiltration membranes. These newly developed membranes have proven to be effective barriers for micropollutants in lab-scale experiments and have recently become commercially available for large-scale applications. However, translating these lab-scale results, which were mainly performed using synthetic water matrices, to real-world applications is not straightforward. Further knowledge needs to be acquired to determine the role these membranes can play in greywater treatment systems.
This thesis investigates the applicability of the polyelectrolyte multilayer (PEM) based hollow fiber nanofiltration membranes as a post-treatment in greywater treatment systems to remove micropollutants. Several key factors, such as the water matrix, need to be considered to effectively implement these newly developed membranes in greywater treatment systems. It is known that the ionic composition of the studied water matrix substantially affects the transport of trace contaminants. To study this known effect for our application, the removal of (un)charged micropollutants in the presence of different ion pairs at environmentally relevant concentrations was investigated in Chapter 2. Substantial changes in charged micropollutant removal by the PEM based nanofiltration were observed as a function of the ionic composition. To elucidate the potential mechanisms leading to these effects, qualitative modeling of the trends was performed, which indicated the occurrence of two mechanisms: changes in the charge-based properties of the membranes and electrostatic coupling of trace micropollutants and ions due to spontaneous transmembrane electric fields.
Following the investigation of the ionic strength, the water matrix's complexity was increased by simulating the greywater effluent matrix and using real greywater effluent collected from a local source separated treatment plant. Fouling can substantially impact the performance of membranes; however, it has received limited attention when using hollow fiber nanofiltration membranes. Therefore, the effect of model foulants and actual greywater effluent on the performance of hollow fiber nanofiltration membranes was investigated in Chapter 3. The experiments demonstrated that the studied hollow fiber nanofiltration membranes are mainly sensitive to biofouling. Permeability loss was observed in all foulant experiments, and overall, sodium alginate most severely reduced the flux. Regardless of the loss in permeability, micropollutant retention remained relatively constant, demonstrating the potential suitability of hollow fiber nanofiltration membranes in greywater reclamation schemes.
While hollow fiber nanofiltration effectively removes micropollutants under normal operational conditions, membrane integrity is crucial to ensure the produced water's safety. To quickly and adequately detect potential fiber failures in a module, several indirect membrane integrity monitoring solutions, consisting of established and novel indicator contaminants, were evaluated in Chapter 4. Clear changes in the removal of organic matter and targeted microorganisms were observed when a single fiber failed in a pilot scale module, while no substantial change in flux was observed. Using the observed changes in retention and a previously proposed hydraulic model, the applicability of the selected indirect monitoring methods was evaluated in upscaled treatment systems. Based on the results, the use of organic matter monitoring via spectroscopy would be the most viable method to ensure membrane integrity in pilot scale systems, while monitoring via microorganisms should be implemented to evaluate large-scale treatment systems.
The work in this thesis culminated in Chapter 5, where a full-scale greywater treatment plant that leverages biological treatment with hollow fiber nanofiltration to remove micropollutants from greywater was investigated. An initial assessment of the entire treatment plant, where both produced permeate and concentrate were discharged, showed the potential of the concept by producing high quality permeate. However, breakthrough of micropollutants to the permeate still occurred. Following this assessment, appropriate disposal of the concentrate by recirculation to the biological treatment instead of discharging was investigated. Finally, low dose ozonation of the concentrate before recirculation to enhance overall micropollutant was studied. While effective removal of a broad group of micropollutants could be achieved and a high quality permeate could be acquired, several substances, such as diclofenac, metformin and galaxolide, still remained in the permeate stream. The results highlighted the potential of the combined greywater treatment system for micropollutants, and eventual water recovery when further permeate polishing is implemented.
Overall, this thesis showed the potential of hollow fiber nanofiltration as an additional treatment step for the removal of micropollutants from greywater. While under controlled conditions high retentions of micropollutants could be achieved, full-scale, integrated implementation of hollow fiber nanofiltration in greywater treatment systems still requires further research. Several topics of interest that deserve further investigations to unlock the potential of hollow fiber nanofiltration were provided in Chapter 6. Here, topics, such as the effects of concentrate recirculation on the biological treatment system, the optimization of the ozonation step, the fate of hardly biodegradable substances, and the further valorization of the permeate were identified and should receive ample attention during future research.
