Concentrating on Cakes - PFAS Removal from Reverse Osmosis Concentrate using Cake Filtration
Jurgen Roman is a PhD student in the Department of Soft matter, Fluidics and Interfaces. (Co)Promotors are prof.dr.ir. W.G.J. van der Meer, dr. J.A. Wood and dr.ir. A.J.B. Kemperman from the Faculty of Science & Technology.
The pollution of water resources by man-made chemicals is increasing the already stressed supply of clean drinking water. One type of chemicals that got a lot of attention over the last 20 years are so-called "per- and polyfluoroalkyl substances", more commonly referred to as PFAS. These chemicals have been detected in the blood of whole populations in several countries, do not break down in nature and have been shown to be damaging to human health. In response to these facts, many governmental bodies have imposed limits on the quantity of PFAS that is allowed in drinking water. However, most drinking water treatment plants were not designed to remove these chemicals. This leads to a need for new methods, preferably using existing/commercial technologies, that can serve as new drinking water treatment methods or extend current drinking water plants.
The subsequent treatment of water by first using reverse osmosis membranes and then adsorption with activated carbon, is a conventional water treatment method. In this thesis the simultaneous (instead of subsequent) treatment of water by reverse osmosis and adsorption is investigated. This is accomplished by introducing the adsorption step in the recirculation of a closed circuit reverse osmosis process. The adsorption is performed in a filter cake to be able to keep the hydraulic resistance of the adsorbent layer low. A proper evaluation of this process requires that each of the components: adsorption of PFAS, cake filtration, adsorption in a filter cake and combined operation are well understood.
In Chapter 2, the possibility of using bio-based materials for PFAS adsorption is discussed. Lignin was the most promising material out of the materials that were tested, with adsorption capacities of 1.6, 4.3 and 5.3 ng/g of PFBA, PFBS and PFOS at environmentally relevant concentrations. By carbonizing lignin the specific surface area was increased from 5 m2/g to between 10 and 140 m2/g, with increases in PFAS adsorption mostly proportional to the increase in surface area. Column studies showed that carbonized lignin was 30 times less effective at removing PFAS than conventional activated carbon. Using simple global warming potential calculations based on the specific surface area of the adsorbent, it was shown that it is not preferable to use carbonized lignin over conventional activated carbon.
Chapter 3 describes the usage of residence time distributions to describe solid-liquid separation in a vertical filter vessel. By making use of the residence time distribution of the liquid phase and an additional lag-time accounting for transport of particulates along the length of the filter element, the whole filtration step can be described. By analysing the residuals of the model, insights can be gained on when during the filtration step filtration mechanisms other than cake filtration occur. The method avoids the arbitrary selection and linearizing of experimental filtration data that is required for the conventional analysis method.
The usage of this filter cake to remove PFAS is discussed in Chapter 4. By altering the method of filtration, the structure of the filter cake is varied to determine the effects of filter cake structure on PFAS adsorption. Precoat filtration showed a ≈20% higher kinetic adsorption of PFAS than simultaneous filtration of filter-aid and adsorption material. Additionally, there was a strong negative influence of the amount of filter aid on the PFAS adsorption. The presence of percolation pathways with a low hydraulic resistance and little adsorption material, formed by the filter aid, is considered to be the reason for the reduced PFAS adsorption.
The combined closed circuit reverse osmosis with adsorption process is investigated in Chapter 5, to treat scaling prone water with PFAS. Using the adsorbent filter cake, it was possible to reach a total recovery of 99\% with only 18\% reduction of the membrane permeability, as most of the scaling was deposited in/on the filter cake. If the filtration is properly performed to form a filter cake with low hydraulic resistance, this low resistance can persist throughout the whole treatment. Simultaneously, the concentrate is treated, whereby most of the PFAS can be removed during operation of the closed circuit reverse osmosis. The process shows viability for concentrate treatment of conventional reverse osmosis plants, to increase water recovery while removing pollutants.
The results from preceding chapters are summarized in Chapter 6 and reflected on their applicability in a broader view. Additionally, some recommendations for further research are given and investigations into alternate operation and configuration of CCRO adsorption are shown.
