Concentrating on cakes: PFAS removal from reverse osmosis concentrate using cake filtration

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. To produce drinking water with PFAS levels below regulatory norms, many current water treatment facilities have to be modified. The subsequent treatment of water by first using reverse osmosis membranes and then adsorption with activated carbon, is an already 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: therefore adsorption of PFAS, cake filtration, adsorption in a filter cake and combined operation are investigated.

In Chapter 1 motivation, scientific and regulatory context for the thesis are given. Chapter 2 discusses the usage of various bio-based materials as adsorbents for PFAS and compares them to conventional activated carbon. Chapter 3, the transient description of cake filtration experiments in a vertical filter vessel is shown. In Chapter 4, the influence of the formation of an adsorption bed by cake filtration on its subsequent use as an adsorbent bed for PFAS is discussed. Subsequently, these elements are combined in Chapter 5 which describes the simultaneous closed circuit reverse osmosis adsorption treatment for scaling prone water to increase water recovery and remove PFAS. Finally in Chapter 6 the results are summarized and reflected on.