Membrane aeration for an effective transfer of oxygen to water

Aeration is a process whose purpose is the transfer of oxygen from a gaseous to a liquid phase. This is an essential operation in water and wastewater treatment to ensure the presence of dissolved oxygen to support chemical or secondary biological processes but unfortunately suffers from a limitation to a mass transfer due to the low solubility of oxygen in water, which imposes the need to use high quantities of excess air or to operate with pure oxygen. To overcome this drawback and improve the aeration process, porous membranes could be an attractive option since they can offer the opportunity to establish a very high interphase contact area devoted to an effective mass transfer. The membranes could work either with or without bubble formation and that feature can be important in the presence of some wastewater of a particular composition and tendency to foaming and in biofilm reactors. This PhD thesis aims to study the aeration process performed by using membranes with and without bubble generation highlighting the strengths and weaknesses of this technology and using both commercial and lab-modified membranes to improve the oxygen transfer to water. The mass transfer coefficient (KLa) and the standard oxygen transfer efficiency (SOTE) were estimated to evaluate the performance of several membrane units prepared with different sizes. In bubble bubble-free membrane aerator the resistance related to the transfer of oxygen into the liquid was identified to be the limiting resistance of the overall process if compared with the one related to the gas and to the membrane. Several parameters (e.g. liquid flowrate, gas flowrate, pressure inside the fibers, membrane surface, membrane thickness) were tested to see their effect on the performance and the thickness of the membrane proved to be very important. These types of aerators were then studied and applied in Membrane Aerated Biofilm Reactors (MABRs) and a very good biofilm development on the membranes was observed resulting in a good abatement of organics and ammonia. The bubble membrane aeration was studied both on a lab scale (e.g. 3 L) and full scale (up to 500L) by performing a comparison with the conventional perforated disk aerators. The membrane showed higher efficiency and could work at significantly lower airflow rates. However, the long-time operation revealed the tendency to scale on these devices' surfaces. To further improve the mass transfer efficiency of the bubble membrane aerator the optimization of the membrane to reduce the bubble dimension and the detachment time becomes necessary. For this purpose, it is fundamental the proper choice of the materials and especially the identification and the realization of the suitable surface properties of the membranes. Hydrophobic surfaces were more prone to form big bubbles with longer detachment time and a marked tendency to coalesce directly at the membrane surface. Higher roughness and surface porosity of membranes, on the contrary, favoured the detachment of the bubbles. The investigation on the influence of membrane properties on bubble formation showed that the membrane should pre-establish a hydrophilic surface in contact with the liquid, to reduce the bubble size and the time of detachment, and a hydrophobic underlying structure to avoid the flooding of the membrane pores. To achieve this, the surface of some commercial membranes composed by different polymers was modified to obtain Janus membranes. The modification was successful and led to an increase of the mass transfer coefficients in the case of the polypropylene hollow fiber membranes.