Antifouling performance of polytetrafluoroethylene and polyvinylidene fluoride ultrafiltration membranes during alkali/surfactant/polymer flooding wastewater treatment: Distinctions and mechanisms

Alkali/surfactant/polymer (ASP) flooding wastewater is highly caustic, and membrane fouling is the main obstacle during ASP ultrafiltration (UF) treatment. To maintain favorable filtration performance, polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF) membranes were implemented here, and their antifouling properties and mechanisms were investigated based on the threshold flux theory. Compared with the PVDF membranes, the PTFE membranes exhibited superior antifouling properties with lower reductions in flux and smaller hydraulic resistance, and they presented a nearly identical pseudo-stable fouling rate at a later time point. In the fouling layers of the PTFE and PVDF membranes, anion polyacrylamide (APAM) was observed along with divalent/trivalent metal ions. The thermodynamic and molecular mechanisms of membrane fouling by APAM were elucidated using the Extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) theory and atomic force microscopy (AFM), respectively. The calculated total interfacial free energy (mJ/m²) of adhesion between the APAM and PTFE membranes was positive, and the value between the APAM and PVDF membranes was negative. Furthermore, the values and interaction distances of the measured intermolecular rupture and approaching forces were larger for APAM-PTFE than for APAM-PVDF. For the PTFE membranes, the positive free energies and smaller intermolecular interaction resulted in weaker APAM-PTFE adhesion and adsorption and therefore the lower levels of flux decline and the later achievement of the pseudo-stable fouling rate. Additionally, the total flux recoveries observed after physical cleaning reached 0.78-0.80 and 0.32-0.39 for the PTFE and PVDF membranes, respectively, which showed that the PTFE membranes can be cleaned easily. The PTFE membranes have considerable potential for extensive application in UF treatments for ASP wastewater. These results should promote understanding the essence of the threshold flux and the fouling control of UF membranes.

Effective treatment of olive mill effluents from two-phase and three-phase extraction processes by batch membranes in series operation upon threshold conditions

Production of olive oil results in the generation of high amounts of heavy polluted effluents characterized by extremely variable contaminants degree, leading to sensible complexity in treatment. In this work, batch membrane processes in series comprising ultrafiltration (UF), nanofiltration (NF) and reverse osmosis (RO) are used to purify the effluents exiting both the two-phase and tree-phase extraction processes to a grade compatible to the discharge in municipal sewer systems in Spain and Italy. However, one main problem in applying this technology to wastewater management issues is given by membrane fouling. In the last years, the threshold flux theory was introduced as a key tool to understand fouling problems, and threshold flux measurement can give valuable information regarding optimal membrane process design and operation. In the present manuscript, mathematical approach of threshold flux conditions for membranes operation is addressed, also implementing proper pretreatment processes such as pH-T flocculation and UV/TiO2 photocatalysis with ferromagnetic-core nanoparticles in order to reduce membranes fouling. Both influence the organic matter content as well as the particle size distribution of the solutes surviving in the wastewater stream, leading, when properly applied, to reduced fouling, higher rejection and recovery values, thus enhancing the economic feasibility of the process.

Microalgal biomass production by using ultra- and nanofiltration membrane fractions of olive mill wastewater

Olive milling produces huge amounts of wastewater (OMWW) characterized by an extremely high organic load. Its polyphenols content is a hindrance to conventional biological treatment and to using it as growing medium for common microbial biomasses. The practice to dump it on soil is in conflict with the latest EU directives about waste management. OMWW can be effectively and efficiently treated by means of membrane technology to a fraction of the initial volume, but membrane processing concentrates still require treatment. Reversing the overall cost balance of membrane processing and subsequent treatment requires valorizing the concentrates through their reuse, as well as ensuring long-term service of the membrane system through effective wastewater pretreatment and sustainable, fouling-controlling, membrane operation conduite. Aim of this work is to reuse and valorize the ultra- and nanofiltration membrane concentrates as media for biomass production of microalgae and cyanobacteria. Scenedesmus dimorphus and Arthrospira platensis, usable as a food, feed, nutraceutical component or feedstock for biofuels, were selected for this investigation. Microalgal growth was experimentally determined and related to the composition of the concentrate-based media and to the irradiance distribution within the photobioreactor volume to decouple light limitation and medium chemical composition effects.