Micro- and ultrafiltration membranes for wastewater treatment and microalgae filtration: influence of membrane properties and operational condition on membrane fouling

Promovendus/a: Lisendra Marbelia

Promotor: Prof. dr. ir. Ivo Vankelecom

The use of membrane for solid-liquid separations has widespread. In this PhD, two applications of micro- and ultrafiltration membrane processes are covered: for wastewater (i.e. activated sludge filtration in an aerobic membrane bioreactor (MBR)) and microalgae filtration. While MBRs already form a mature technology compared to the use of membrane technology in microalgae processes, membrane-feed interactions are inevitable and challenging in both applications. This phenomenon, known as membrane fouling, is manifested in a lower permeate flux during filtration.

The ultimate aim of this thesis is to give a better insight in membrane fouling for activated sludge and microalgae filtration, and to define strategies for fouling mitigation by exploiting the membrane properties and the filtration parameters. Three main steps were taken in order to realize the aforementioned aim: (1) synthesis or use and characterization of membranes with different properties (i.e. materials, porosity and surface porosity, surface charge, hydrophilicity and surface pattern), (2) filtration of activated sludge (MBRs) or various microalgae species with different set-ups and filtration condition (i.e. flux, filtration mode, aeration, vibration, and applied chemical cleaning) and (3) filtration and fouling analysis, mainly based on permeances and trans membrane pressure (TMP) values and physico-chemical characterization of the fouled membranes. Finally, the inter-correlation between those three points was analyzed.

In this first part of this research, PVDF lab-made membranes prepared via phase inversion and commercial PVC/silica mixed matrix membranes were used to study the fouling phenomena in MBRs. PVDF membranes were produced with different properties, i.e. pore size, porosity, and clean water permeance, by altering synthesis parameters. The influence of three phase inversion parameters (polymer concentration, additive, and evaporation time) on the membrane properties was investigated and then linked to the fouling behavior. Results show that more porous membranes show their superiority through a higher critical flux and (slightly) higher sustained permeance in long-term tests. However, the advantage of these porous membranes vanishes in quite a short period due to severe pore blocking of the largest pores. For both membrane materials, it is clear that too big pores are not beneficial and isoporous membranes might offer better filtration performance.

In addition to membrane porosity, the influence of membrane surface patterning and vibrations on MBR performance were also investigated using flat and ribbed PVC/silica mixed matrix membranes. Two main advantages are offered by the ribbed membranes: higher clean water permeability due to additional active filtering area coming from the ribs, and a thick membrane with good mechanical integrity. In the vibration mode, membrane fouling was decreased both with the flat and the ribbed membrane. However, the current dimension of the ribs in combination with the applied vibration didn’t give any improvement. Thus, further study to scale down the dimension of the ribs and to optimize the cross-flow velocity near the membrane surface is needed.

In the second part of this research, the use of a microalgae photobioreactor (PBR) was investigated for polishing the permeate of MBRs. The membrane in the PBR was able to integrate both microalgae cultivation and pre-harvesting. In comparison to the conventional PBRs, the MPBR allowed higher dilution rates to be applied, thus resulting in higher biomass concentrations and higher productivities. Nutrients could also be taken up by the microalgae in both PBR and MPBR. However, nutrient removal efficiency went down with increasing dilution rate, meaning that a balance should be found between a good biomass productivity with a decent nutrient removal efficiency.

The third part of this research focused on membrane applications for microalgae filtration. Membranes based on PAN with different porosity and surface charge were exploited in this research to filter different microalgae species. Different filterability of these species was shown in both dead-end and submerged filtration, which could be linked to the cake resistance values which are influenced by cell shape and size, cell-wall rigidity, and probably also TEP content. A more hydrophilic surface of modified membranes induced a less membrane-solutes interactions and resulted in lower fouling rates in the beginning of the filtration. However, the benefits of the hydrophilic membranes seemed to evade with filtration time, indicating the dominance of the formed cake layer in the overall performance.